What makes you think it's the amp. Is the headunit still working but no sound? I've read a few threads here and I've experienced myself where the fiber optic cable needed to be replace. Check that before the amp
I borrowed an amp from a 911 (997.645.334.33) and it worked on a few of the speakers (not surprising as I have a sub woofer and it is a different part number)and with my original unit nothing at all so it is definitely the amp that is faulty, cheers