I have the same problem, i also have a STR-DE715. I have two thunder acoustic speakers and two 12in. punch subs. Now when i only play the subs i can go max volume, no problem, great bass the whole deal. Then when i play the thunder speakers i cant even go half way without the receiver shutting off and saying "protector" on the screen(both the speakers are 120 watt). From personal experience i think it could be the speakers youre using, either the receiver is giving too much juice and its preventing the speakers from blowing or the speakers need more power and the receiver cant give it. So a new set of speaker might be what you need. The receiver is 280 watts so something in that zone should work out. Also try turning off the sound field if you're using more than two speakers for the front output (i read this in the manual, you can pull up the users manual on www.manualshark.org
and see if you can find something i might've missed) it supposed tp help it in one way or another. E-mail me at email@example.com
and tell me if your problem was fixed or not. Im pretty curious myself to find a solution to this problem. hope this helps!