I have a pair of JBL 4412s and am currently waiting for a Yamaha P3500s amplifier to arrive in order to power them.

These speakers have an RMS rating of 150 watts and the amp I have bought is rated at 375 watts (at 8 ohms). The problem I have the speakers, but the amp is going to take two or three weeks to come into the country.

In the mean time I thought I would try powering the speakers with an old cheap amp my parents had lying around (A pioneer sx-p520, which is an integrated amp from an all in one system.). I don't know what the rating of this amp is, but the sticker on the back of the speakers that would normally be powered by this amp says max power 40 watts, so I assume the amp isn't very powerful.

Anyway, I hooked it up and the sound is there but it is quite muffled, like a wet towel has been placed over the speakers. I am being careful not to clip the signal, but still I can make the sound quite loud, although the dynamics in the amplitude do seem to be limited. Given my speakers are new and haven't been broken in, my question is this constrained sound a product of the speakers not getting enough power, or something else (like them not being broken in or quality of the amp etc) and can driving speakers without enough power be damaging to the speakers (as long as they don't clip)?

Thanks