Jas1 since you have been doing some voltage testing I would suggest one other. Check the AC voltage at the speaker terminals while playing loud passages or test tones. My speakers are pretty inefficient, dipping to 3.6 Ohms at certain frequencies an I have never had a heating problem even after many hours of loud playing. While playing LOUD I have measured the voltages by placing the meter across the biding posts and setting the meter to capture the highest voltage. This loud playing will ussually give reading peaks about 20 VAC which works out to around 100 watts or 5 amps. Doing this may give you some idea of how much current individual speakers are pulling which may in turn help diagnose the problem. You could also use this method to check at the speakers first and then at the amp to see if the wiring is adding much of a load.

If this process of diagnosing is flawed I am sure someone will post with corrections