Mordaunt-Short MS907W subwoofer: I switched speakers and now I get no sound. The unit powers on and the standby mode works fine. Sratchy, muffled bass sound comes out. But nothing from the speakers. I tested with some speaker wire to bridge from the in-coming cable to the out-going. The speakers worked. So, the signal is going into the unit but not coming out to the speakers. What could be wrong?