It sounds like only *one* of the speaker wire conductors is shorted to house/earth ground within the wall (either conductor) and would cause the problems he's describing depending how his amps are configured. Note how the Kenwood works fine on the suspect wires. This grounding issue could affect other components feeding the amp via the interconnects and shared AC grounding, thus potentially shorting input circuits.
If the speaker cables were terminated in metal wall boxes (or not), disconnect the speakers and amp from the wires, connect your meter to one of wire conductors and the other meter lead to the nearest AC out metal cover screw, or the outlet's "ground" contact. Note any meter shorting, then try the wire's other conductor to outlet's "ground" conductor. If wall outlets are to far away, use a 3-prong extension cord and repeat the steps above to the cord's "ground", or any other grounded metal piping or electrical conduit.
If the speaker cables were terminated in metal wall boxes (or not), disconnect the speakers and amp from the wires, connect your meter to one of wire conductors and the other meter lead to the nearest AC out metal cover screw, or the outlet's "ground" contact. Note any meter shorting, then try the wire's other conductor to outlet's "ground" conductor. If wall outlets are to far away, use a 3-prong extension cord and repeat the steps above to the cord's "ground", or any other grounded metal piping or electrical conduit.