Telex EVsonicXS thermal fuse???

Hi guys, I hope someone here can help a newbie out. I've recently bought a set of Telex EVsonicXS 4.1 speakers for my PC and after working fine for a few weeks they just completely stopped working all of a sudden. I checked all the obvious stuff (sound card, cable connections, power supply, etc.) and could find nothing that would have caused them to quit.

The manual states that prolonged use at excessively high volume levels could cause a thermal fuse to blow which would require out of warranty service. The amp for these speakers was always warmer than I thought it should have been and didn't really seem any warmer than any other time I have checked it, but since I did have the volume slightly over halfway up and had it that way for some time I'm guessing that I overheated things and blew the thermal fuse. I've tried contacting the service number in the manual and was told that they no longer service Telex equipment. I've tried e-mailing the Telex computer audio division twice with no response, so I guess it's up to me to fix it or else I'm stuck with some expensive paper weights. Fixing it is going to be a challenge because I don't have a schematic so I'm doing a lot of guessing.

I've pulled the amp out of the subwoofer case and the only fuse I have found is the main power fuse in the back of the case which I checked anyway and isn't blown. There is power getting to the amp, and the LED on the main satellite speaker is lit so there is power going from the amp to the satellites. From the research I've done I think that this amp must have some sort of protection (the "thermal fuse" mentioned in the manual) on the audio output side of the amp. Does this sound right? The amp has five separate boards (one for each satellite and one for the sub), that are attached to one main board. Since there is no audio output to any of the speakers I assume that the thermal fuse is located on the main board but I have seen nothing on the board resembling a fuse and no obvious sign of overheating.

Now is when I am really guessing so bear with me on this and if I'm way off base please let me know. The only component that I can find that I suspect is a mosfet that is attached to a heat sink on the front corner of the board. Since this is the only part that is attached to a heatsink I expect it to be the part they figured was the most susceptable to heat related failure and the location is easy to get to which also seemed logical for a part that they obviously expected to have to replace or they wouldn't have mentioned it in the manual. The part number on this is SD669A. This doesn't fit my image of a "thermal fuse" but I'm thinking that it could serve a similar function. Am I correct in this assumption? If so, how can I test it to see if it is functioning properly. If I'm completely out in left field on this does anyone have any suggestions on what else I should be checking?

Thanks for the help!