Sounds like a sub amp problem to me. If you really want to try a repair, remove the amp and closely examine the power supply filter capacitors for bulging or leaking. You may also have a leaky transistor in the pre-amp section of the sub amp. None of this is for the uninitiated. Trade it in when you can.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated