Greetings, I own both the Pathos Classic One MkII as well as the Naim Nait 5i - both are very respectable. But they do very different things!
The Naim is very "flat earth" sounding; lots of detail and nothing horribly wrong, but definitely not warm sounding. More like a studio sound.
The Pathos is rich, extremely inviting sounding. I liked the sound of it in stereo mode, but adding a second one and using them in bridged mode is exceptional!
I put the Naim Nait 5i into the system for a comparison between it and the bridged Pathos amps. I could only take it for about five minutes. Lifeless, flat. Very uninvolving in comparison. This is not to bash the Naim, but it was not as exhilirating as the twin Pathos' combo.
I am using the Naim at my office now and if one did not know what other amps sounded like, they'd say it sounds great. I know there is extreme devotion to Naim amps in some circles. I am keeping the naim for the office, and it's a sweetie, but I would think that a bit of sweetness with the Pathos would be the ticket for the B&W speakers. The Naim would likely make them sound a bit clinical (detailed, but lacking the warmth).
You can get a Pathos Classic One MkII or the Naim Nait 5i both for under $1,400. If you're a maniac like me, you'd buy both and demo them against each other, then sell off the loser. I have not heard the Pathos Classic One MkIII, but it would be substantially more. In fact, you might be able to purchase two MkII's and bridge them like me for about the price of one MkIII! In bridged mode, the Classic One moves up into another whole category of sound. It is by far the best ampification I have tried to date.
Kindly take a look at my system, judge for yourself if you think my comments are valid.