But it won't like being fed 400W of power either, no matter how clean the signal.

Right, but the point is that if you turn the amp gains down very low, and make sure not to turn things up so loud that you clip the speakers, then it's not sending 400w to the speakers. You could drive 30w speakers off of a 400w amp without damaging them. As long as you're careful.

I mean, yes, you could also melt the speakers quite quickly if you weren't careful. It certainly would be harder to get a 30w amp to melt speakers than a 400w amp. But the 400w amp doesn't necessarily have to be deadly as long as it's turned down.

Oh, and as long as it never "pops". A 400w "pop" could certainly be deadly for some factory speakers. Hmm, hadn't thought of that...