Sorry, I can't answer the query but I have exactly the same issue and tried all sorts of things to be certain it really is the amp and have concluded it is. Did you get anywhere with this? If you sent it to be fixed what happened i.e. was it expensive, what was the problem etc. Loathe to bin it as it's been great and to replace a system as good would cost me too much.
Hope you can help
aidlesultan, December 2010