First, I would think you need to know what it is that is affecting the sound. In amps they talk about clipping, distortion, slew rates and the like as affecting sound quality.
Now, you can measure the difference in distortion between two amps (if you have the right equipment), but how do you know that specifically is audibly affecting the sound the amp produces?
I seem to remember Tom Cumberland talking about slew rates and cliping when I asked him about the A1400 and sound quality. I keep hoping that Alan will jump in to one of these discussions as I think that he is probably the best one on the boards to address this topic.
