So, that Weather Report track (from Heavy Weather on Columbia). In moments, I  was utterly gripped. Fast, detailed, and insightful, with explosive transients and that beautiful tonal colour and tunefulness that Sugden does so well. The bass was tight and melodic, the percussion fast and urgent, and the sax’s phrasing was absolutely on the money. I remember going to a Weather Report gig in my youth, not long after Heavy Weather came out, and this was as fresh and exciting an experience as I remember from that day. Good hi-fi is about evoking feelings, generating a visceral, emotional response to your music, and the Masterclass ANV-50 was absolutely delivering right at that very moment.

It’s not perfect, and while the 50-Watt output (doubling to 100 Watts into a 4 Ohm load, which is itself quite a rare capacity in the real world) gives it considerably more than twice the headroom and grunt of the A21, it can’t quite match the solid bottom end of my (twice as expensive) 110-Watt Albarry M1108s. So that famous opening bass riff of ‘Birdland’ has power, impact, and tunefulness, but just misses out on a degree of mass and solidity – Pastorius’ bass goes ‘boing’ not ‘Boing!’, but you’d only notice if you’d heard it differently elsewhere. In all other respects, the freshness and vitality of the piece just leapt out of the speakers, aided no doubt by the sort of natural timing and ‘rightness’ that just doesn’t draw attention to itself. This was the music speaking, not the system.

Setting to one side, for a moment, thoughts of lower bass, I turned to the more contemplative and thoughtful Tord Gustavsen Ensemble, specifically ‘Wrapped in a yielding air’ from Restored Returned [ECM] which, while seldom rising above a mezzo forte still manages a sinuous, compelling propulsion in the twists and turns of its phrasing and the understated percussion. Its textures, particularly Kristin Asbjørnsen’s characterful vocals, are a key part of the whole performance, and the ANV-50 delivered its complex layers and subtleties impeccably. And, as it happens, bass really isn’t an issue. Tord Gustavsen again, but this time with his trio on ‘At Home’ from Being There [ECM]; there is a skilful and oh so subtle build up of energy and urgency. Again, the volume seldom rises above moderate levels, and lesser amps struggle to portray the energy if there isn’t much of yer actual loudness, but the ANV-50 absolutely nailed it. It’s a clever trick, depicting energy without loudness, and it takes a very accomplished amp to pull it off. ‘Vesper’ from the same album showed that the amp is more than capable of giving a convincing account of the weight of a piano’s lower registers, with a solid, convincing, and above all, tuneful bass.

The Sugden Masterclass ANV-50 reminds me of another of my very favourite amplifiers, the Lavardin IS Reference. Both have a speed, delicacy, and subtlety, a way with textures, phrasing, and timing that simply elevates their performance above their peers. The Sugden, however, also has ample reserves of power and drive, despite its 50-Watt output not being far removed from the Lavardin’s nominal 45 Watts. The difference, I suspect, is how the Sugden deals with lower impedance or reactive loads, which neither the Lavardin, nor Sugden’s own A21 can even approach.

So, the on-paper power output means the ANV-50 isn’t a heavy-hitter, but the delivery is robust enough for it not to show under most circumstances. Billy Taylor’s piano on ‘I wish I knew how it would feel to be free’ from Music keeps us young (Arkadia) can indeed be as weighty and powerful as you expect, if you turn the wick up a bit, but sometimes that comes at the expense of a small degree of the piece’s signature bounce, and the subtlest phrasing touches.

These are very small trade-offs, however, and the truly remarkable thing is that I’m nit-picking because there’s very little else. And in truth, any small nits I’ve picked are largely I suspect down to the relatively modest power output, rather than any issues with the amp itself. Probably fair to say that 50 Watt, doubling into a 4Ω load, is going to be more than adequate for most applications.

I use Russell K Red 150 floorstanders for most of my reviewing; these are a fairly straightforward load, but not especially efficient at 87dB for 1 Watt at 1 metre. Thoroughly antisocial volume levels were entirely possible in my typical UK domestic-sized listening room, but a 50-Watt amp is never going to move as much air as a big ‘un, and it was when extended that the limits of the power did show. The Royal Concertgebouw Orchestra’s account of the Dvorˆák 9th Symphony [RCO Live, SACD] has huge reserves of energy, and dynamics to pin you in your seat. But it’s not lacking in subtlety. The Masterclass ANV-50 delivered nearly all the energy and drive you could wish for, for example in the Scherzo third movement and the finale, but if I drove it hard there was a slight trade off in resolution. The tone and pitch of timpani was perhaps a touch vague, compared to the Albarries, and there was a small reduction in the scale of the soundstage and depth. But the ANV-50 didn’t lose its composure or become congested, and this is a piece which has foxed many an amplifier in my system. Played at a slightly lower level, the spatial detail and texture was all there, so I conclude that any limitations are largely down to the ultimate reserves of power you have available. Some of it may be the preamp section (the Sugden has a pre-out option) which is a little leaner and a shade less emphatic than my Albarry, but the essential character of the amplifier as a whole was consistent, whether I used just the pre- or used it as an integrated design.

The amp uses its available power as effectively as any amplifier I’ve heard. As I mentioned before, it manages that neat trick of portraying the energy in the music without needing to be playing loud, so it’s an absolute wonder at creating tension and drama, and letting the musicians’ skills shine through in the subtlety of their playing and phrasing. All these little micro-dynamic details which lesser amps are apt to airbrush away can so often make the difference between an enjoyable performance, and a truly memorable one. The Sugden just owns it. Take Joanna MacGregor playing ‘Autumn in Warsaw’ from Play [Sound Circus]; this is a subtle, yet complex and hugely technically demanding piece, and MacGregor’s articulation and phrasing is finely wrought. It goes loud, but the musical forces are not massive; here the piano has very good weight, scale and solidity, and the build up of tension and drama is so skilfully managed that the apocalyptic climax to the piece is all the more effective in its impact. It’s about the way the power is delivered – just the right amount, at just the right moment.

So, timing is another particular strength; the late, and much missed Jacques Loussier’s ‘Pastorale’ from Plays Bach[Telarc] has lots of subtle inner detail and little delights – the interplay between the piano and the percussion with its peculiar ‘not quite off-beat’ counterpoint, or the way piano and bass work together and, in particular the melodic and tuneful nature of the acoustic bass. Marc Cohn’s ‘Ghost Train’ from Marc Cohn [Atlantic] showed off its inner detail and rhythmic structure, the tom toms and tambourine being particularly tactile and texturally rich; but it’s subtly done, not pushed into your face, and all the more effective for it.

The Sugden Masterclass ANV-50 is, by any standards, a remarkable achievement. As a spiritual, if not literal successor to the pioneering A21, the amp that made Sugden’s name, it has some big shoes to fill. On this experience, it does so magnificently. It is light on its feet, and has a deftness that eludes so many otherwise decent amplifiers. Coupled with a perfectly respectable and usable power output, and the reserves to double that output into difficult loads, it’s going to be quite a challenge to find anything at the price that will give you anything like the musical satisfaction and sheer joy this amplifier delivers. Here’s to the next fifty years.