Actually DSP's do make a difference. The ability to handle more than one tasks is what can set them apart. I'm 100% in agreement that 99.9999% of the people would not be able to tell one amplifier to another. I also maintain that any decent speaker would not be distinguishable in an A/B test.
But Marantz's DSP for example, cannot handle TrueHD, DTS-MA *and* apply Audyssey room correction. Denon's DSPs can. Some AVRs do an improper job of boosting LFE by 10dB do to FW bugs. So when you take an AVR (not just the amplifier portion) as a package, one can (and do) sound different. It may have to do with FW bugs, DSPs that lack horsepower etc, but to an end user it will "sound" different.
(By AVR Package, I mean all the features that come with it)
