REL No.31 Subwoofer REVIEW


This is a review written by Terry Ellis August 2023.

For my video review please see my YouTube channel link here

Where Do You Start

Reviewing subwoofers, especially big heavy and expensive ones like a pair of REL No.31 is not easy when you don’t have an obvious reference point. Now you could take the subs out into a field, run them to their maximum and measure their output and distortion using ground plane and that data can be very useful, but it can also be misleading if you don’t understand the data, or if you only look at it in insolation.

Then when you put subwoofers into a room everything changes anyway and there is a different metric of assessment happening which is always, eventually,  a subjective one. Then setting up subwoofers in rooms you can be a very technical and precise affair, I do this all the time as part of my Dirac calibration work and so I understand the importance of this. Normally this technical process is essential if you want to get the bass just right.

What is crazy about this review is that the No.31 are quite large subwoofers. For my video review I wanted to make sure I could get them on camera easily and still be able to get to all the kit on the racks behind them and still have plants on show. For the subwoofer installation we (too heavy for one man to lift) literally just plonked them down where they are now in the photos / video. I moved the left one forward about two inches after listening to them, but I haven’t moved apart from that.  I was expecting, in some regards, lessened bass performance and maybe limited integration with whatever speakers are in the blue taped position but, to my amazement, I have never had or heard better integrated subwoofers in this room than these REL 31s, and never experienced the same level of overall system performance uplift from other subwoofers either.

These subwoofers are incredible for music and gone are the worries of whether the subwoofers will keep up with the speakers. Using the 31s you are worried if your speakers will keep up with the subwoofers. This might all sound like hyperbole but it’s all true.

Specification and Design

The No.31 are REL’s current “smaller” flagship subwoofer and they are thirty-one years in the making, hence the name. They feature REL’s current best 12 inch carbon-fibre diaphragm driver, with a limited to 900 watt class D amplifier. They are a sealed design with a very special cabinet that’s almost tear drop in shape when the grills are on.  I never normally have grilles on anything, the REL grilles extend the front curvature and make the subwoofers look even less of a square box. The cabinets are lovely looking by subwoofer standards and exude quality. There are nicely sculpted curved recesses for the handles, grooves cut into the top, a nice footing system and the carbon REL plaque on top – all touches of class. Die-hard performance enthusiasts might turn their nose up at these eccentricities, but let me reassure you, when these subwoofers are visible in your room day after day these details matter, and they definitely fuel the pride of ownership part of your emotions.

They also have a large footprint and so make sure you measure at least twice before you order them because they are a significant amount bigger than RELs other subwoofers. Fortunately, bigger is better when it comes to subwoofer performance.

On the rear is a typical REL subwoofer amplifier plate which has a lot of connections for high level, line level and more. However, you will see something missing and that is controls. The No.31 subwoofers come with a very lovely and well made carbon-esque remote control that helpfully allows you to set up the subwoofers from the listening position. I do apologise to nearly every other REL subwoofer owner as this is a far better way to setup subwoofers than controls on the rear. It is almost on par with app control, different of course. Using the remote you can adjust the phase, the high level crossover, volume level for both the high level and .1, and the two built in parametric equalisers. You can see the corresponding values displayed on a screen in the top right corner of the subwoofer. You do have to point the remote at the subwoofer you want to adjust and this can be a little finicky as sometimes you adjust both and not just one sub. In a way this feels old school compared to app control but there is also something I like in the tactile feel of holding a big heavy high quality remote control.

For clarification I have only used the REL’s exclusively for two channel music listening, so no home theatre and I have been using their high-level connection, which is a signal feed straight from the amplifier’s output. For this review I am using REL’s Bass Line Blue subwoofer cables.

Preliminary Testing and Sound Quality

Testing the RELs was interesting for me because I had to disconnect them from my system many times so that I could review individual things like speakers or amplifiers, but then I had to reconnect the RELs to review them. Through this process I was able to hear their effect and benefit to several system component combinations with different speakers. Every single time I added the RELs the system sounded massively, night and day better, no exaggeration or bullshit, the systems sounded exponentially better every time.

How you setup subwoofers will massively dictate how they sound, and I already mentioned about their placement. I overplayed this a little, I know my listening room very well and have measured speakers and subwoofer pretty much all over it so I knew the subs would be ok placed where they are, but I still wasn’t expecting such a truly excellent bass from them positioned here. I also had help at the start from REL’s own Rob Hunt who did an initial setup when he delivered them. You might have seen Rob feature in videos I have made, one is linked here. His job is REL subwoofers, and he has a real knack with setting them up well very quickly. He set up the 31s in quite a traditional and neutral way crossing over quite low at around 30hz, which is where the Mission 770, the speakers I was using at the time, were audibly rolling off. The REL were transparent and filled the room with low bass ridiculously easily, they were barely on because my room is naturally hot in that region due to its smaller size. Yet it was very surprising to me how much sonic difference the REL made to the overall system sound even at that low crossover frequency and volume level, showing there is a lot of deep bass information in music which many of us never hear or get to appreciate.

I like a lot of bass in music and the warmer, fuller, and more punchy sound it creates. Of course, I also needed to know what the RELs could do and so I set about setting them up myself. I ended up crossing over around 46hz and with the left sub slightly higher in volume than the right as that better balanced the pressure in the room. Being able to do that is a huge benefit of having dual subs. I also set the RELs much louder in volume than Rob had done as I wanted more of a “ballsy” sound. Interestingly the 31s would have easily done more in terms of output at higher bass frequencies still but that messed up the integration or balance with the main speakers, or maybe my room couldn’t handle that much pressure with both the speakers and subwoofers playing and so I hit a threshold of how much bass I could have. That threshold then kind of set the standard for the speakers to match up to, which is very interesting when you think about it, and why I said at the beginning the speakers need to keep up.

Now one very important thing to talk about here is the effectiveness and benefit of the built in two equalisers.  Turning the bass up for more impact in the mid 40hz’s region inevitably meant a lot more in the 30hz and below region also again because of my rooms dimensions.  I used EQ1 to reduce 30hz by enough to bring the deepest bass back to being tight sounding in the room again and not excessive and this made it possible to achieve the bass I wanted and not have to reduce the sub level because to a bass I am forced to have.  That to me is priceless if I am not using a more advanced system like Dirac Live. 

Effect in Various Systems

The first system I tested the 31s with was my Mission 770 speakers and the Synthesis Roma AC510, which is a “high” power tube amplifier which drove the Missions easily. Listening to that system you would say it has a full sound with very good bass and yet, when you add the No. 31’s, you realise that’s not the case. Adding the RELs firstly gave a much greater sense of scale and it sounded like listening to a much bigger and a much more expensive system which, of course, it now was. However, adding the 31s didn’t just extend the sound from the speakers into the lower registers, it created a different sounding system, one that sounded much better and more high end.

You may be wondering how simply adding subwoofers can do that – well the extra bass helps to fill the room with warm pressure when the music asks for it but, more importantly, it helps to solidify each element of the music. It doesn’t matter if it’s a double bass, where you would expect a subwoofer to do that, but it also has an effect with drums, guitar, synth and, most impressively, the extra richness of tone and fullness to both male and female vocals. The extra bass pressure warmth and solidity helps to abate the systems sense of stress so that everything sounds more RELaxed (geddit!), which is ultimately much more enjoyable, enveloping, and immersive.  So, adding the REL subs is not just about adding more or extending the bass, there are other sonic knock-on benefits that happen psychoacoustically.

Next, I tested the RELs with the Galion TS120SE tube amplifier through the Mission 770 and then £22,000 Sonus Faber Serafino speakers. The Galion tube amp again delivers excellent bass and, when you listen in isolation, you would be more than happy but, after you hear the 31’s do their thing, they made the system sound much bigger, bolder, richer, and more solid, with greater scale and immersion. Again, essentially making the system sound much more expensive and expansive with all music. Take them away and the system sound feelt empty, hollow, and two and a half dimensional for soundstage, which is a mind bender as just before I thought the system sounded very three dimensional, but once I heard the RELs dramatic effect the removal of them left a big hole here.

My takeaway from this is that if you are using tube amps, even very good ones which deliver great bass, subwoofers, especially ones this good, will likely be immense for your system’s sound.

I made the last test deliberately much tougher for the RELs as I brought in the big power McIntosh MA352 hybrid integrated, which has 320 watts of power into the 4-ohm Sonus Faber Serafino. This combination drove the Sonus Fabers better than the Galion tube amp in terms of bass output, scale, and authority, making them sound almost subwoofer like already. I was therefore expecting there to be much less effect from adding the 31s but, to my amazement, the RELs made even more of a positive difference here. I am guessing they blended better with the higher performing Serafino. After thinking about it, I realised that this is because the Serafino were now keeping up better with the REL, which is crazy to think about but true, as I heard it. This pairing together with the rest of the system I have currently in my listening room has given me the best sound I have ever had in the room, and I wouldn’t say that about this system without the RELs. Take from that what you will.

Bigger is Better

To this point, the differences I have referred to will, I think, be delivered by most good subwoofers if they are setup well. Definitely other RELs will, the T, S and the Carbon Special all do this, and they all give you a much better sound in the same kind of way.  Why would you spend the extra? The No.31 cost £7,000 each so that is £14,000 worth of subwoofers, which is a lot of money.

I will do my best to explain what I think the differences are. I have had several other subwoofers in my listening room from REL and other brands, but I have never had bass this good. I have previously compared the REL S510 subs to a REL T9X and the S510 seemed to produce a more voluptuous sounding bass.  Deeper, smoother more rounded and muscial.  The No.31 do that too but much more again. They have the big, rounded, bold character of a large subwoofer and yet the lightness of touch of a smaller one. You notice this playing with speakers as good as the Sonus Faber Serafino. You can tell the subwoofers are playing because of the amount bass pressure you can feel you can’t tell what bass note is coming from the speakers and what is from the subwoofers. The no.31s therefore seem as fast and transparent as dual 7 inch bass drivers in speakers that cost £22K and yet they play much louder and much deeper. I don’t really think you can ask for much more than that from a subwoofer for ma HiFi system.

I love how the bass from the 31s is big, full, rich and has real impact and throb factor but with no blurring so that you can hear subtleties in the bass like I have not experienced before from subwoofers especially with no advanced room correction being used before them.

When I listen to guilty-pleasure music like some Sub Focus, commercial drum and bass I really like that the bass is now substantial enough for this music. It is not from just the speakers alone, as they don’t put out enough of the deep throbby bass to do this club intended music justice. Yet changing to a Hans Zimmer movie soundtrack, it doesn’t matter which movie, the RELs give the same kind of bass scale you get from a home theatre system when that music is the key emotional moment of the movie. You again don’t get that from the just the speakers so the emotion of the musical score is not delivered in the same way.  Then put on something more cultured, like classical or some female jazz vocalists, or even some Skillet rock, and RELs help elevate the sense of drama, scale, impact, and presence you again don’t get from the speakers alone, all with clarity, gracefulness, and a pleasing warmth.


I love what the No. 31s are doing in my system, and I love them as a HiFi product and what they do. They are awesome, and I can’t think of any other word to describe them. It’s not hyperbole, they are legitimately awesome musical subwoofers.

The REL No. 31s are not cheap, and they are big. As I’ve said before, big is good when it comes to subwoofers, but your significant other may not agree. Also, the more bass you feed into your room the more bass bleeds out into other rooms in the house which something to consider in the real world of practicality, but who cares about that when you only live once.

If you are a big-numbers on paper, deep-bass crazy, extreme subwoofer-performance orientated individual, yes you could buy many dual 18inch subwoofers for the £14K asking price and, of course, those will move a lot more air and might suit a different end goal better.

Instead if you are looking to underpin speakers in a HiFi system I am yet to experience better than these REL 31s. In this room at least they really are awesome (there’s that word again) and REL will have a fight on their hands to get them back from me, that damn emotional attachment we audiophiles develop to black boxes.

pecial Performer Award Website No Background

A Special Performer Award is Pursuit Perfect Systems highest accolade and is in recognition of exceptional product performance regardless of price

For the full specification of the REL No.31 subwoofers please see their website linked here