Serious Serie S/510 REL Acoustics subwoofers REVIEW

rel s510 subwoofers review website 1

This is a review written by Terry Ellis July 2022.

For my video review please see my YouTube channel link here

For me bass is the most important part of any audio system because it effects everything else so much. Some of you might be thinking no it’s the vocals and mid-range that is most important.  To try and demonstrate my point of view, do this simple test. Take a piece of music, use the tone controls on your amp and turn the bass up a little bit. It’s likely the music now sounds better in ways, a little warmer with vocals sounding fuller and more pleasing.  Now turn the bass control all the way off and the life of the music, the beat, the rhythm, the soul disappears and the solid vocal you were just enjoying now sounds sound empty and hollow.  For me, bass from 500hz and down is the most important part to get right for any audio system (Dirac Calibration first rule) but it’s also the hardest part to get right because of many factors that are out of your control.

With subwoofers we are looking at sub-bass frequencies, maybe 100hz and below. However, what’s interesting is, I have been to many REL subwoofer demos where I have heard how their subs  help to improve the sound of vocals, the perception of sound stage width and depth, and sometimes even treble can sound better, more pleasing. I think this has a lot to do with the tone controls demo we started with and how we experience music when there is more deep bass, I have written an article about this before. Maybe there is more to it, REL believe so.

Either way, with all these demos and all the systems I have calibrated with Dirac, I am confident enough to sit here and say I believe 99% of audio systems will benefit from well setup subwoofers. But what if you are a HiFi lover who feels that they are part of the 1% and don’t need a subwoofer?

That is what I put to the test reviewing a pair of REL Series S 5/10 subwoofers.

Some Background

I have had the REL S/510 subs here for several months but haven’t always been able to always use them. Not because I lacked the desire to, but because I am always trying to isolate the product I am reviewing as much as possible, if that product is speakers I feel they should be reviewed in isolation with subwoofers in the main.  Regulars to my YouTube channel will hopefully remember back to the start of the year when I reviewed the TAD Compact EVO 1 speakers. They were the first speakers I used when I changed my room from the black bat cave cinema room, to a more focused on 2 channel lighter room – video here. I struggled to get the bass I wanted from the TAD CE1 and some of that was me not having them in the best location in my room for bass, I also feel some of it was the character and tuning of the speakers combined with the sound and setup of the HiFi system I had at the time.

In this instance adding the two RELs was a night and day improvement. I was able to use them to get the bass exactly how I wanted it to be, and I was impressed with how fast, detailed, lively and punchy the RELs were.  I was able to integrate the RELs seamlessly with the TAD so that you couldn’t hear where the speaker finished, and the subwoofers started. The TAD CE1 are £20k speakers and the two baby S range RELs were more than capable of keeping up with them and made all the difference in that system.

When I look back at that situation, it was a relatively easy one for the RELs because it was a prime situation where the speakers on their own are not producing the bass required and this is a very common situation, much more common than is realised.

Several Months Later

Moving on several months, I am now much more familiar with my new room layout and the difference in the acoustics. I have found a more optimum positioning for speakers three-dimensional imaging and their bass. This, as ever, still a compromise because where the bass is best is not best for the imaging, but its a better compromise than before. I am using Dirac more effectively to maximise and minimise what the room is doing to the speakers sound for the full frequency band but particularly in the bass as that is where most negative effects happen.  On top of that I have been reviewing a great amplifier the Audiozen Embrace and have made some other improvements to my system, mostly notably the JCAT OPTIMO S ATX linear power supply.  The result of all this has meant I have been enjoying my best ever sound in my listening room.

I am using the new Mission 770 which are stunningly good speakers which on paper only deliver bass down to about 40 hz but, in practice, in this room, they are delivering measured bass performance down to 20hz because of room boundary gain.  I have dialled their sound into my room using Dirac to within an inch of perfection, or the best I can.  At this point I didn’t really feel like I needed the REL subwoofers in the system, the sound from the speakers alone was totally satisfying to me, and the measured bass response in the room down to 20hz what more do you need than that for bass extension.  I genuinely thought I was one of the 1% whose systems might not see as huge benefit from adding subwoofers.

Of course, we are talking two REL S/510 subwoofers here, at £2,299, each so their combined cost is more than the speakers.  This is fine because you can’t have a perfect HiFi sound without perfect bass, its logical common sense.

Subwoofers with Style

For £2,299 we do get very handsome looking subwoofers with a very high quality gloss paint finish.  I think they look really cool in my room. I am normally a go commando audiophile when it comes to grilles and the REL grilles are big, chunky and heavy, very good quality. The subs take on a different look with the grilles on that might be more your thing. I’m on the fence with these subs if I prefer the grilles-on look. On the other hand, there is something I really like about seeing the silver drivers in action. The best thing is that on-or off, the subs look great.

The S/510 are compact subwoofers by the standards of a lot of modern designs making them more domestically acceptable, high WAF. The side panel design includes contrasting handles which is a little indulgent but, in the flesh, looks good and does break up the typical black box look.

The S/510 feature a 10 inch continuous cast aluminium alloy actively driven main driver, and underneath is a 12 inch passive radiator that has a large amount of linear travel for better performance, the performance of a larger driver whan called for. The system is driven by a 500w Class D amplifier. You can buy much larger and more powerful subwoofers for the same price, if that is what you are looking for, even REL sell larger and more powerful subwoofers for less money the HT/1508 Predators. At one time I had four of them in this room and they their total cost too was not that much more.

But they didn’t have RELs subwoofer USP their high level connection which is the subwoofer taking its signal directly from the amplifiers speaker output, feeding into what REL call their Perfectfilters.  These are in essence very fast low pass filter with the main goal of the subwoofer being able to deliver sub bass frequencies with the same speed as the main speakers being used. This is why REL don’t use DSP in their subs they believe their analogue filter to be superior in the time domain. It is the high-level connection which make REL subs easier to use with all audio systems, but especially HiFi systems as there is no need for a dedicated line level subwoofer output to be present

The last few things to mention are the rail style feet which make it possible to safely stack the S/510 up to three high to create a six pack of subwoofer. I have experienced this before and would love to again, its mightily impressive in ways you don’t expect. The plants would have to go, of course.  For this review I have used the RELs with the REL Bass Line Blue high level subwoofer cables exclusively for two channel use, and not home cinema. REL say that these are their best cables for the job. I much prefer their spade connection at the amplifier end because it is much easier to cable them and feels more secure. I have never been a big fan of the normal REL high level cable because of its bare wire at the amplifier ended.  I think they would better with a spade termination for the same reasons ease of use and security of connection.

Were there any sonic benefits? Lots

Coming back to my 1% situation. Just to recap, I have my HiFi system really dialled in and I am totally satisfied with the overall sound. I know the bass is not perfect because that is generally a dream, very rarely a reality because of the room acoustic complexity, however, there are no obvious shortcomings after my Dirac calibration and the bass was very satisfying

Time to find out what happens when I add the two RELs and set them up to integrate with the Mission 770. Not unsurprisingly, pretty much the same thing happened as when I added them to TAD CE1. Initially I want to mention the benefit of the RELs for vinyl playback because I was quite amazed by this.  I played a record produced by speaker manufacturer Dali that I was fortunate to be gifted the album at the recent Munich Hi-End show.  The record called “Let the Good time come” and  is available from the Dali Store here 

This is a record well worth buying, not only because it is audiophile grade but it is also good music, something we don’t come across very often on a test quality record.  It is a nice mixture of blues and folk, but with a modern sound, some guitar, big male vocals and some bass. It is not record that screams subwoofers at you but listening and testing between the RELs on and off, the difference with them on was huge. There was more body and soul to the singers’ voice, more overall presence, grandiose and scale with more fullness, warmth and richness to everything.  I have to say that this album, in my system and room, made the entry ‘ish’ level AVID Ingenium turntable sound not entry level at all.  What the RELs were adding was really making a huge difference by filling in a lot of what was obviously missing when I turned them off.  Subjectively I much preferred the sound of this album with the RELs in the system and, interestingly, there wasn’t the one main negative side effect of adding subwoofers which was I getting with more listening to my digital setup.

With my digital setup the system has a much better starting point than my vinyl setup because of what I can do with Dirac.  I can undo a lot of the my room’s negative influence on the bass of the Missions at my listening position and the influence of Dirac is enormous on the overall sound of the system.  Its the difference between OK and excellent.  So what happened when I added the RELs to the system for digital playback, well pretty much the same again.  Straight away vocals sounded better, more full and solid for both male and female singers.  Overall, the system sounded more complete, more full from top to bottom. There was more information apparent than before which is interesting because it is increased bass information helping to complete the musical picture happening in front of you. The information was mostly there with the Missions on their own, but more complete with the RELs added, an easily audible improvement.

Then there were the obvious benefits such as more power and presence to the bass in anything from rap music to pipe organ. There was more power and presence of that bass in the room which, I found more pleasing and realistic, we are supposed to feel pipe organs as much as hear them.  I cannot say there was improved sound staging adding the RELs, the Mission 770 speakers were doing excellent in that regard given the limitations of my room size. I also didn’t find an improvement to treble because, again, the system was already very good before, very well balanced before but now with the improved bass I think it would be easier to add and balance super tweeters, which is something I am interested to explore at some point.

From Subjective to Objective Measurement

I said at the start of the review bass is the most important factor to get right in any audio system but also the hardest due to external factors like the listening room and its acoustics. To this point all my opinions have been subjective,  what about some objective data see below.  I measured the speakers at the MLP in my room and the overall response from the Mission 770 is pretty good,  but it doesn’t look perfect because it’s not,  that is the reality of what bass is like from speakers in a room, the room messes things up big time.  You can see some significant bass nulls at 60hz and 40hz which is far from ideal but extremely common.  When I add the RELs you can see the bass nulls go away. That is one indicator of why I was hearing improved vocals, more information, and a fuller more complete sound from the bottom to the top.  I did all these measurements after listening, because that is important and looking at the graphs things are still far from perfect.  I understand because of what textbooks show you, aggressively smoothed frequency plots this looks like there would be so much bass it would be unbearable to listen but that is far from the case.  There was some bass excess in some music where the notes are ringing in my room, and these are always ringing and because of the extra bass energy of the subwoofers its more audible.  That is because the subs are outputting much more energy at deeper bass frequencies than the speakers are, which is a good thing, and what a subwoofer is supposed to do. However, it does bring me to a very important point that you cannot just add subwoofers and expect perfect bass glory.

Subwoofers do take some setting up, beyond getting the phase right, sub bass frequencies, 100hz and down are all huge sound waves that bounce around your room causing bass problems. Subwoofer setup, or bass setup in general, is critical but difficult to get right, especially just by ear.  REL have made several videos about how to setup their subwoofers and I would recommend you check those out for some assistance.  I have my own way of doing it using Dirac. For me that gives me the most control and the best end results.  That is why the subwoofers look very hot in the graphs because that is how they sounded best for the upper sub bass regions and I used Dirac to precisely manage the regions of bass where there was excess.  Obviously you could just turn the subwoofer volume down to make the excess bass less obvious but that would be counter productive for the bass level through the crossover.  With Dirac I can have the best of both and better for both.

I can say its much easier to integrate subwoofers into a HiFi system when you have two of them rather than just one. There are many technical reasons and benefits for having multiple subwoofers in a room and you can easily find information on this. To simplify things,  I set up different AV and HiFi systems all the time with some of the best tools available and it is always easier, and you generally get better results, with two or more subwoofers. Please take my comments as good advice, its is not about selling someone more products, I am not a dealer I don’t sell anything, its just good advice.


I’d like to tell you about a question I asked REL.  I reviewed REL’s T9X subwoofer late last year and thought it was a great subwoofer for the money.  I asked REL why would someone buy 2 S/510 instead 2 T9X at around double the price? The answer was, twice the power so more control and more speed so the user will get more of what RELs can do for their system”, referring to sound stage and improvements I mentioned earlier.

I think the Serie S subwoofers from REL sound bigger, bolder, and more “voluptuous” than their T (inc TX) subs. This is maybe because they have more power and therefore have more output at deeper frequencies.  This is just an observation I have made after I have played with loads of REL’s subwoofer in a lot of systems.  REL have a speaker pairing tool on the website. If you are unsure, it is worth looking on there for some advice.

REL have made a believer out me for high level connected subwoofers and I wouldn’t have said that a couple of years ago. I think the S/510 are probably designed for rooms like mine – smaller to medium size but with the speed and quality to match very good speakers.  Other subwoofers can perhaps do the cinema job better at this price point, especially if you are chasing big output at low frequencies as much as kicking as you can get per pound. I don’t feel dissatisfied with the S//510 in anyway for two channel HiFi use, and I have heard them do some great cinema bass in other systems as well.

But are they the right subwoofers for you and your system, I think with all of these important system decisions, you have to balance what do you need, what do you have, and what’s your budget.  The S/510 are going to cost you a serious chunk money for a pair of them, but they are  good enough to support very good speakers likely improving the sound of the system in ways you cannot achieve without them, even with the best DSP and patience of a saint. As opposed to previously being 99% certain, I am now 99.99% certain that subwoofers can benefit all audio systems.

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For the full specification of the REL S/510 please see their website linked here