Did you do a quick run over on all the connectors leading to the speakers? Might be a loose solder on the speakers themselves, perhaps at a certain volume, the connection is lost or is crossing, causing the short. I'd also check the specs on the stereo, see if the amperage is distributed correctly, do you have an amp? If so, check it's capability and connections. If not, then have you considered the possibility that you might not have enough amperage from your stereo itself?
Just some thoughts I had running through my head about past experiences I've had with electronics in general.