It's very squeaky clean. Strengths are tonal quality and imaging - it's beautiful and airy like a tube amp while retaining some class A/B grunt. It has timing that surprises you since usually good PRAT comes with a tonal quality hit, and this amp avoids it. In short a very successful and rare example of a machine that was designed to capture some of the advantages of a tube amp but on a muscular, solid-state platform. It also has two balanced XLR inputs and a pre-out RCA connector so it's versatile. Essentially it's a tube preamp and a solid state power amp in one box.
It's weaknesses are few and minor but of interest if you are thinking of switching from something as good as any Sugden Masterclass. It has, as I said, some good solid-state grunt but at it's price, it can only have so much. Don't think you're getting grunt in the class of the Plinius/McIntosh/Krell world (the amps I refer to are much more expensive obviously...but just saying, don't don't get up hope that they've cheated the audio gods with the hybrid thing). The other thing is, Pathos make several hybrid amps in which both stages - power and pre - are running in pure class A, not just the tube preamp like in the Logos. These "INPOL" amps are the real result of the Pathos obsession with the hybrid.
Thus I think the INPOL based amps from Pathos are more comparable to a Sugden power amp hooked to a tube preamp. These would be an "upgrade" from most Sugden setups in that they aim to do something similar, better, and suceed, at times brilliantly.
But if you are thinking of moving from the Sugden Masterclass series - wonderful Class A amps - then perhaps the itch you are trying to scratch is you want more of that grunt. In that case I see why the Logos appeals to you - you can hang on to some of your Sugdeny sound while gaining some Real Juice.
If that is the case I would not necesarily discourage the Pathos, but just encourage you to listen to some alternatives. The Logos would have similar sound quality to your current amp - maybe a little more natural and holographic if it pairs well with your Harbeths, which I have not heard - with a little more grunt. I suggest that what you might want to try is a LOT more grunt - might be more satisfying. And once you get into Pathos price territory you can buy some damn good sound quality with your large heat sink. Never underestimate the ability of massive amounts of power to make music sound great, overcoming other factors.
The reason I am happy with the Logos is that I appreciate that engineering is a compromise. No-compromise products (which really mean, making huge compromises on everything else so you can be "no compromise" on whatever your schtick is) are easier to market and sell because they are more exciting. A product like the Logos is much more Continental in character - it specifically seeks a character, at a specific price, of a product that really competes fairly well on everything, without blowing anyone away on anything. I admire this and think that my amp is almost as good as a tube amp on character, almost as good as a pure class A on tonality, and almost as good as a big hunk of metal on PRAT. This type of design excites me; I believe it is brilliant. It will be better than your Sugden in every respect except the 1-2 things your Sugden is best at (in which it will be worse but still pretty good).
Whether this excites YOU or not depends on the type of system you are looking for, and how you want to feel over the coming years when you listen to it every day.
Good luck with it - hope this helps. I've been lurking here for years - finally posted.