High pitched whine - unless Source Direct

Dear All,

I have three Cyrus power amplifiers, which I was using with a Cyrus AV8 pre-amp.

The AV8 has stopped sending out a signal, one by one the channels dropped out of use, until now none of them work.

So I bought a Marantz SR7300 receiver, and have connected it to the three Cyrus power amps.

Once I had got everything connected nicely, and seemingly working, a high-pitched whine appeared, at what I make to be about 1500Hz.

My first thought was ground loop, so I reconfigured the power sources and the earthing, but it didn't change anything.

Today I discovered that if, instead of using any of the sound modes offered by the receiver, I use "direct source" - the whine goes away. However, this mode removes surround sound and subwoofer, and more importantly it can't play the optical signal from the TV. If I select "direct source" while listening to TV there is no whine, but no audio either.

Does anybody know to get around this? I feel like I am close to mission successful here, but I may be wrong. If it involves replacing the capacitors I would be willing to try.

Thanks for your suggestions!