Try substituting some source components out of the system one at a time. For instance, try a source other than your CD rig (if you don't have a tuner, tape deck, or phono, then use a VCR or DVD), try running a source directly into your amp bypassing your preamp (watch out for too high levels, though - use a quiet source material), try everything without the PLC, etc. If none of that reveals where the problem is, try to put another power amp or receiver of some kind in place of the amp. You may also want to recheck all of your cable connections and cable dressing routes throughout the system. Process of elemination, in other words. Good luck!