Pass Labs INT25 V Sugden Masterclass ANV50

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I am currently reviewing the Pearl Acoustics Sibelius speakers. They are an interesting and unique design because they are a single driver speaker with no crossover. That means no filter in between the speaker and the amplifier, except maybe the cables, but ignoring cables, the Sibelius is surely a speaker which will let you hear an amplifier’s character, if it has one.  As part of the review, I wanted to make sure I tested the Sibelius speakers with a few quality amplifiers. Class A has long been thought to be ‘the best’ and two of them are, at heart, Class A designs. But is Class A ‘the dream’ in 2022? It has been fascinating to listen to the different sounds of Class A amplification.

The Sibelius is a speaker that has a history, a meaning, and a reason for existing, and so there is a lot to talk about. The purpose of this article is to discuss the amplifiers I have partnered with the Sibelius, I feel many of you may well be interested in these particular amps. This is not a full review of these amplifiers either as I have not done extensive enough testing. It is just a look at how these amps sound through the lens that is the Sibelius.

Pass Labs INT25

First up is Pass Labs, a classic HiFi brand from the US that needs no introduction. The INT25 is the smallest of the Pass Labs Class A topology integrated amplifiers and it delivers 25 watts per channel into 8 ohms. This is my first time listening to one of their amplifiers in my system.  It is a very traditional looking amplifier with a rugged construction and massive heatsinks on the sides. This certainly gives it that high-end amplifier look and it makes you think of Krell, or similar. The build quality is excellent, and it is quite heavy. I particularly like the aluminium remote control and so those important premium high end expectation boxes are ticked.

There is one big thing missing for me here though, and that is the famous Pass Labs meter on the front, which I always thought was a normal VU meter but, by all accounts, they are ‘current consumption’ meters, which is different but, I think, equally cool to feature on the front of an amplifier.  The price here in the UK for the INT25 is £7,200, which is a serious chunk of amplifier investment.

Pass Labs say on their website that the INT25 was able to be a more simplistic design because of its low power output. They have also scaled back the inputs to three single ended and no balanced because it’s a simpler single ended amplifier design, Tube audiophiles take note here.

After giving the amp sufficient time to fully warm up I started my critical listening through the Sibelius. I was provided with a wonderfully inviting sweet sound which was extremely musical, by that I mean music flows with an effortless and graceful nature. The sound is smooth, maybe almost too smooth for some audiophiles, but I think that all audiophiles will find the INT25 captivating in the mid band because it is so lush and clean, regardless of the content. If you want to know what the word euphonic sounds like, seek out this amplifier for a listen and it will take you deep into music in a quite unique way. This will be addictive for some by creating a wonderful soundscape and just grabbing your emotions. The amplifier is a genuine pleasure to listen to.

Vocals have great tonality, combined with a very open soundstage, and that is always a winning combination. For me the INT25 with the Sibelius lacks a little drama and bite, everything is just so smooth and graceful so music like dance or electronica is wonderfully euphoric, but can lack some impact, especially in the bass. Don’t get me wrong the bass is there and it’s nice, warm and lush, but lacks some attack or just bass control, I think this could be better. Turning up the volume certainly helped but didn’t seem to be the answer. I need to be careful here with my criticism because, in the grander scheme of things, the sound of the Pass Labs is extremely impressive and, for some audiophiles with different priorities, they could easily love the sound of this pairing.

Sugden Masterclass ANV50

Next up to test with the Sibelius speakers is another very interesting amplifier. This time it is a design from the UK, the Sugden Masterclass ANV50. What’s great here is that Sugden is another famous HiFi manufacturer with a pedigree in Class A amplifier design.  Sugden say that they built the world’s first pure Class A solid state amplifier in 1967, which is a significant claim to HiFi fame. They say that the ANV50 is, at its heart, a pure Class A amplifier, but one designed with more modern living in mind, such as running much cooler and only using 20 watts of power at idle.

The ANV50 can deliver a healthy amount of power – 50 watts into 8 ohms per channel and 100 watts into 4 ohms.  That is double the power of the Pass Labs and the Sugden costs around half as much at about £4,500, depending on the retailer.  It offers a lot more connections than the Pass as well. The overall build quality maybe doesn’t feel quite as substantial, perhaps because the amplifier is light.  The chassis is large, but quite hollow, and the metalwork doesn’t feel as generous. However, all the button and knobs feel high quality, and so I am likely just splitting hairs here.  The Sugden remote control is plastic and certainly doesn’t feel as premium as the Pass Labs metal remote. When you look under the hood, the ANV50 couldn’t be any more different in design to the Pass Labs either.

Dissecting the difference

Although both amplifiers are Class A based, this does not mean that they sound the same or even similar. In fact, they sound totally different.  Straight away when driving the Sibelius with the Sugden you can hear the speakers take advantage of the extra power available as they deliver a more energetic sound at lower volumes, with a better articulated bass.  The sound character is also the opposite to the Pass Labs as it is more forward into the room, with a greater degree of emphasis on the high frequencies. That doesn’t mean bright, it means that higher frequencies are just clearer and have more presence, contrasting with the Pass Labs which sounds a little softer in the highs and, as I discovered, also in the lows.  The Sugden sounds very precise, with an open soundstage and, depending on the music, it has a livelier character and works better with electronic music.  However, for me, there is something missing from the amplifier’s presentation with the Sibelius and that is the vocals and mid range.  The Sibelius have an obviously leaner character than with the Pass Labs and I found it too lean to properly engage me when listening to ‘big’ vocalists.  I was also detecting a little bit of vocal excess, or over excitement, of lower vocals with the Sugden, which I wasn’t getting from the Pass Labs. That made me question whether it could be the power being too much for the speaker. This is something I clearly needed to test further and later when I measued the speakers I did spot something that could have been the cause, but either way its what I heard at the time.  I also wanted to see if I could find a sound which was the best of both amplifiers, or as close to as possible.

That is where I think a real-world issue comes in for Class A amplification. I am sure you can have the best of both in a Class A amplifier design but, no doubt, at a cost because the big power Class A amplifiers that I can think of that can achieve this are much more expensive, such as from the Gryphon with their Essence, or maybe the dual Canor Audio AI1.10 amplifiers I reviewed a year or so ago. Both solutions will cost you double and more. Of course, Pass Labs have their larger more powerful Class A integrate amps, the INT60 and INT250 at around £9,000 and £11,500 respectively. I don’t know if the larger INT250 is a pure Class A amplifier either, so again real-world limitations come into play here. I have no issue with the suggestion of using amplifiers that cost more than the speakers as amplifiers are hugely important for sound, but it is potentially a lot of money we are talking about here.

Best of Both, almost

The final option I tested was the Leema Acoustics Tucana II Anniversary, which at around £5,500 offered me close to what I was looking for. In terms of power, the Leema delivers, by far, the largest power output of 150w per channel into 8 ohms and 290 watts into 4 ohms. You immediately hear that in the bass delivery from the Sibelius.  It is similar for output to the Sugden, but the bass is tighter and more percussive.  The Sibelius have a limit to what they can output in terms of sound pressure levels’ especially for bass. The Leema is easily able to put you nicely on that limit and keep you there, with no adverse effect.  Interestingly, the Leema is not an amplifier I would call dark sounding at all but, compared to the Sugden, it does sound darker or just toned down in the higher frequencies.  There is a little loss of sound stage air or ambience presence as a result, but the trade-off for a more solid and tonally saturated vocal is worth it to me because voices have a more natural timbre, which makes them more pleasing to listen to. There is also more energy, drive and grip to the music with the Leema compared to the Pass Labs. I am not sure the Leema is as sweet sounding as the Pass Labs and not nearly as euphonic, instead it is certainly more straight forward and, maybe, more honest about its musical delivery, and so the Pass Labs seems to have an obviously pleasing character, which I do like, but we trade that off for control, grip and authority, a characteristic which I also really like and got from the Leema Tucana in spades.  


I like the Pass Labs with the Sibelius speakers. In fact, it is hard to not like an amplifier with this kind of character, and I like it more than the Sugden for the vocal delivery, tonality, richness and warmth it imbues to the music. In some ways I prefer the Sugden for the energy and life it provides to the higher frequencies and bass delivery but, of the two, I would choose the Pass Labs.

What’s interesting here is I think that the best overall amplifier, with the Sibelius speakers, for the music I listen to, especially for the money and at the volumes I listen at, in my system and in my acoustically treated listening room, (a lot of caveats there!) is the Leema Acoustics Tucana II Anniversary. This is interesting because it’s a Class A/B design.

This demonstrates to me yet again that, with audio design, implementation can be more important than just the specific technology used. To me, that is an important outcome because, as an audiophile, it is easy to become obsessed with specific things, such as wanting pure Class A amplifier because you think it will be best. After conducting this comparison, I now understand why class A might not not always the best choice in every situation, especially at this kind of price level of amplifier. 

What has clearly come from this is how important it is to get the right amplifier for your speakers, what audiophiles refer to as synergy. I can envisage Sibelius owners maybe owning more than one amplifier in order to give them the flexibility of different sounds for different moods or musical interests they have at that time.