Tim, this is rather unlikely, but in your speaker wire connections there could be a loose strand that shorts out only at high volume. If you haven't already done so, check all the connections and carefully re-do them even if they look okay. Those speakers don't present an unusually tough load, and that in itself shouldn't be the cause of the problem. Try disconnecting one speaker at a time and playing the system without it to see if possibly one of the speakers has an internal wiring problem and needs to be replaced.
