First, things like "liveliness" and the like are effects that are much more energetic than the measured accuracy of very many amplifiers. If an amplifier has actually different sensation at that level, either it, or it's pair in the comparison, is broken, full stop. This will show up in an in-situ measurement like a blinding searchlight. Now, make sure your comparisons are level matched, because that kind of "feel" is often attributed to something that is a pure level difference.
As to the "damping factor" thing, almost all modern speakers are built with the idea that the amplifier is very close to a pure voltage source. As such, there really should be no effects of that sort AT ALL. If not, again, something is just plain broken.