Worf -- Stan's response led me to realize why you were asking your question about damaging speakers by playing at low volume. Keep in mind that playing a high powered amp at low volume is a very different situation than playing an underpowered amp at a volume level that is more than it can handle (that exceeds its power rating).
In the first case the speakers are fed a clean, undistorted signal. In the second case, as Stan indicates, the speaker may be fed a highly distorted (clipped) signal. It is the clipping distortion that can do the damage, not the low volume.
Clipping distortion will sound like the music is breaking up on peaks. It will have a static-like character whenever the music gets loud.
Stan -- A small correction to your explanation. The damage would not be caused by the clipped waveform causing dc to flow. The danger with a clipped waveform is the sharp transition points where the normal sine wave turns into a flat top or flat bottom. The abrupt transition points contain high frequency spectral components that are not present in the original waveform. Their high frequency causes the speaker crossovers to route them to the tweeters, causing the tweeters to have to handle abnormally high power levels, which leads to overheating and burn-out.