Originally Posted by bjaurelio
Question 1: If both amplifiers are properly designed with a flat frequency response and low distortion, you will not hear a difference.
Question 2: Aside from clipping, amplifiers usually have increased distortion when pushed at higher output. In reality, for home use, you're probably rarely using more than a few watts. Just because your new speakers say they are rated for 300+ watts does not mean you need that much to power them. Those B&W speakers are rated at 90 db at 2.83V. With a nominal impedance of 8 ohms and minimum of 3 ohms, you're looking at 1-2 watts giving you 90 db, presumably at the standard 1 meter for sensitivity measurements. Just to make sure you understand, to get 90 db at 3 feet away from the speaker, you only need 1-2 watts of power for those speakers. Even if you want to hit 100 db peaks, that's only requiring 10-20 watts of power. This is all assuming accurate manufacturer specifications. TLDR, a 200 WPC amp is more than sufficient for the B&W speakers you are auditioning.
I haven't read everything to the end, so this may have been talked about already.
Amps usually produce the cleanest signal just before clipping, as in say 99% output (very close to the rail voltage). This is because the noise floor basically stays the same, but the output increases, which lowers your THD+N (since it's seen in relation to the output signal). Tubes behave differently, they distort more, the harder you drive them.
Just yesterday I did a small test with my 10KW PA amp. I sent a 1khz sine wave to the amp, and back through a voltage divider net into my audio interface. THD was around 0.01% just before clipping (output of a single channel at 106V). When overdriving by ~0.1db, THD jumped to 0.2%. The difference was audible at higher volumes with only this single clean sine wave being played. It became audible at lower levels when reaching about 0.5% THD iirc. I haven't noted anything down because I actually did these tests to determine limiter settings.
Anyways, when working with high sensitivity speakers, noise floor is an important thing to pay attention to. When working with low sensitivity speakers, you basically just need more power to get to a certain level. The low sensitivity will mask many imperfections of an amp (which may produce a (audible) high noise floor).
But those points have been adressed before, just giving my opinion here to back it up. I think either
did some ABX amp tests where they found all DUTs to be indistinguishable. If it wasn't one you, please forgive me!