Say I have a pair of speakers (M60s?) playing "average" rock or classical music in an average room at an undemanding level of volume - one which you could talk over and which wouldn't disturb the neighbors. Would I notice an apperciable difference with a higher-powered amplifier over a lower-powered one (like a NAD C370 vs. a NAD C350)?

From my experience the difference you could hear is the weight and impact of the bass. It would have a bit more definition. I have fiddled around with swapping out amps and receivers and this is what I always notice at the volume level you are talking about.

Having a couple of similar amps (NAD 20w and 40w) that I swap in and out of a midfi setup, I can tell you that there is less of a difference between brand mates, then amps of different manufacturers. The main differences are as mentioned previously, the impact of the frequency response. The larger amp will be able to utilize the increased power (current, etc) when called for, even in average listening.

The basic tenet of this is to compare amplifier sections of similar quality pieces. There is a couple of threads that deal with "quality of amplifier output". I would suggest you search for them.

Regards,

BBIBH