Sometime you can 'feel' a bad voice coil in a speaker by cranking it up medium-high and then with the speaker cradled in both hands, press gently and gradually down on the inner portion of the cone with your thumbs. (Don't poke a hole in the cone!) If the voice coil is cooked, you will feel a spot where it cuts in/out. Other than this test, swapping speakers is a good test...
