Why are DC signals bad for loud speakers?
ALL current will heat the voice coil of a speaker. But AC current is useful to reproduce sounds (which is what a speaker is made for). 

On the other hand, DC current will produce the equivalent amount of heating as an equivalent AC current, but it will produce nothing but a fixed offset (versus moving the cone in and out to produce sound).  And while you can hear AC current, and you can hear when it is "too loud" and distorting the speaker, you cannot hear DC, so you don't know whether your speaker voice-coil is sitting there frying until you see the smoke.Also DC current biases the cone off center which could increase even harmonic distortion.

For these reasons it is never a good idea to allow DC current to go into a speaker voice-coil.