Subs stinking/smoking/distorting/etc. Clipping happens when you ask the amp to produce more power than it can. The peaks of the AC wave are "clipped" off creating DC current, which causes the voice coil to really heat up.
Yes. Lets say you've got your HU at volume 40 and the amp is putting out its full RMS output (say 1000rms) everything is great. You turn it up to 45 and the amp is now trying to put out 1500rms, which it can't. The AC wave is clipped and your subs are seeing DC current, which burns them up quickly.