Accuphase Power Amps

I'd also add 'room size/volume' as being another determining factor.

The room is 17 feet x 14 feet x 11 feet high ceiling. Not sure what that is in CC.
Simon

Simon,
Gathering from all your infos thus far, namely : room size, listening distance, music choice, speaker's sensitivity, and last but not least, coming forth from a Bryston 300w amp.. I too as Dave would, sacrifice that last vestige of refinement of class-A for the more superior all-rounder, ie. put my money on the P4200.

Simon,
Your Bryston is approximately 5db higher in power than the P-4200, or 10db higher than the A-35. I used an online calculator, that's available on numerous websites, with your speaker's sensitivity, listening distance, and power of the Bryston, and determined you currently have the capability for dynamic SPL peaks of 105db. Therefore, the P-4200 would have capability of 100db, the A-35 would be 95db, and the calculator confirmed this. Keep in mind that a dynamic SPL peak can be much higher than average SPL listening volume.

So, at your listening volume, what are the dynamic peak SPL's? Hard to say without measuring. It might be worth getting a sound level meter, and if you do, try both weighting scales and fast response.

Based on your comment about the Telarc recording, as the dynamic peaks could be as much as 25db higher than average listening level, I would get the P-4200. Again, I suggest you borrow or get a well reviewed sound level meter, certainly less than $100, and measure your current system. That way, you would know for sure, and the meter may be useful in the future.

*Note that Accuphase usually conservatively rate their amps' power output.
Mine rated at 100w/8ohm does 180-200w without breaking a sweat, peaking near 300w! (Speaker's minimum impedance is 5ohm).
So assuming similar, you should be expecting close to double the published specs.