Open the BM6500. On the left side is the preamp, on the right side the poweramp and the power supply.
Preamp and power amp are connected by a 3-pin wire, the connector is P4, the wire colours are green, black and yellow. Exchange green and yellow, it could work by mounting the plug 180 degrees... but dont break it.
If the defect continues and does not change the sides, then it's the power amp. If the defect changes to the other side, then it's the pre amp.
If it is the preamp, then probably some capacitors are gone. If you want an audiophil upgrade (... and repair...) , look for the workshop BM7000 on the workbench section.
If it is the amp... ok, that could be a defect power transistor, but this is much more difficult to find.
Martin