I think you might have a bad cd player. You know that the tape input works because you said the Onkyo cassette deck sounds okay. Try puting the cd player into the tape input using the same cable that hooks up the tape deck, (that will eliminate a bad cable). If it still sounds bad, your cd player is at fault.
The tuner, aux and tape inputs are all the same, so it really doesn't matter which one you use. The phono input is for turntables ONLY, do not use it for anything else.
The tuner, aux and tape inputs are all the same, so it really doesn't matter which one you use. The phono input is for turntables ONLY, do not use it for anything else.