Hi CV,
It's easier to do such a comparison in stereo, and it's possible with a Radio Shack line-level switcher. I don't know the model. If you really want to do it right, you should have a switcher with relay switching but that gets expensive. And you also want to have some means to adjust the gain of each amp so they are identical. Otherwise, you'll pick the louder of the two sources as "better" because it sounds louder.
Be careful because with the Radio Shack el-cheapo switcher, you might get switching "thumps" (transients) when you go from A to B.
Using stereo is far better to do any amplifier or speaker comparisons because it's much easier to detect subtle tonal differences and other nuances. There's less going on in the room in terms of room effects, reverberation and so on.
The more channels and speakers that are running, the harder it is to detect meaningful audible differences between speakers and components.
Regards,
