The first thing I'd check is whether or not you have a "dead-short" between speaker wires! This will cause the receiver to go into a shutdown/"protection" mode in most models. Might check that. Also, yes, did you change speakers? Do you run the speakers as "full range?" Might check all this. Receivers have limited power output mostly, and most often than not "small" is the better speaker setting, sending the demanding bass to the subwoofer. Check it all. If all else fails, send the unit for inspection.
Receiver shutting down during high loads
I have a UK made Sherwood Newcastle R-925 receiver that recently started shutting down in surround modes (Dolby Digital and Pro Logic just for testing purposes) under high loads. It worked fine for years and now a gunshot, scream, etc seems to overload the system causing shutdown. As I am not a technician, what could be the problem? I like this unit and would like to have it repaired, but I'm not sure how pricey it may be...