In reply to:
play the same song on a Yamaha, Denon, Sony followed by a B&K separate set up; the difference in sound for music is night and day, even though the same speakers were being used
Were they all in "direct stereo" mode? Were the speaker levels calibrated the same on each amp? Was the listening level the same?
It is actually quite painstaking to do a fair A/B test between amplifiers or speakers. For amps, you must ensure ALL DSP is disabled, and the listening level is the same.
Even simple DSP functions like digital time arrival could be calibrated differently between the amps, altering the sound. On amps with built-in EQ that can make a huge difference. Thus the easiest way is just disable it.
Even then there are differences in how MUCH is disabled in various amps "direct" or "pure direct" mode.
Imagine comparing a Yamaha receiver with EQ (which is well hidden in the menu system) to a non-EQ amp and attributing the sound difference to the amp brand when in reality it's the configuration.
The same with volume. It's well proven that people interpret the louder level as "better", even for small differences.
Getting all these things right is sufficiently laborious that I'll bet many people don't do it. And that's just for stereo. For comparing different amps in a 5.1 config, that's even more complex.
