Disclaimer: All I know about hi fi and pa rigs comes from killing them in my earlier years and learning that sparks and squealing noises are probably a bad sign.
That said, check connections on the back of the amp to ensure that no loose strands of wire are shorting some where and that all connections between amp and source are secure. Other things I would do if it were me and the problem were not the obvious wire one would include:
If the signal is being processed by your Amplifiers Digital to Analog Converter, comparing the hum on a surround sound track as opposed to playing a stereo source through all speakers using the multi channel function. I don't mean using dolby pro logic or similar to reproduce a surround effect but the setting on your amp that plays stereo through the fronts and sends exactly the same signal to your surround speakers. That would, I'm guessing isolate the problem to either the amplification, the source player or the processor.
Also try connecting a source to your amp via the old style analog connections rather than a digital co-ax or fibre optic connection. The signal is then handled by the on board dac of the dvd player or cd not the amp. There by isolating the amps dac entirely. Are you using the amps dac or your dvd player to do the surround processing? Please answer, I'm curious now
All that aside, a channel specific hum is not good. If it's not down to a bad connection/short somewhere then back to the shop with it forthwith. On two amps I've had that developed that kind of hum one was terminal failure of the power amp, the other a short in the pre amp. Both died.
edited for spelling, can't be arsed to make it less garbled though, read it through a couple of times and it'll make more sense, I hope.
