I would try a hard reset, but I'm not optimistic it will work. It seems the power amp on that channel has likely developed a serious fault.
A receiver that old is not worth the cost of repair. In any event there is a high chance parts would not be available.
The service manual is available on line. It is 227 pages long. I note the power amps are ICE amps. These are very difficult to service, and this is where your problem probably lies. This trouble shooting section starts on page 61. Only a very experienced tech, with training on these amps, could begin to set about servicing it.
So, likely if a reset does not do the trick, then this unit should be recycled and it is new receiver time. The good news is that the state of the art has progressed way beyond the technology of 13 years ago.