so your not getting sound from your coaxles/componets? Take the speakers out but leave the wireing connected, then use a volt/ohm meter to test the connection WITH THE STEREO ON. Check all the speakers youre having problems with. If the speakers are recieving signal, but still not playing, then theyre probably busted. If the speakers are busted, use a KOWN WORKING speaker to test the signal again. If your not getting any signal whatsoever, then, yes, you probably have a problem with the amp. Remember, the HU has an amplifier too, this could be the problem.
Thats all Ive got for now. Post again after you check it all out, and give some more information