Sony 5.1 DVD player fault?

I have a Sony HCD SA30 5.1 Surround sound system.
When the volume is turned up, there is a crackling on the speakers, that gets very annoying. I have tried swapping speakers from from to rear, and it appears to do the fault on all speakers on all sockets, thus proving it is the unit itself, not the speakers.
I suspect a thermal problem in the amp, or possibly dry joints in the audio output stages.
Anybody help?
sidthepop@fsmail.net