Use a cable from the Sony surround amp ( front channel out ) to feed the Cary amp, that way only the Cary will have speaker cables run to your ProAc's. It should be the better sounding of the two amps anyway.
If your surround processor is outside of the Sony amp, run a set of cables from the "front" signal out of that processor to an input on your preamp ( I use one titled Video ). Set the preamp to a predetermined volume and when watching movies, return the volume to the same place each time. You will no doubt have to re calibrate your surround system when these changes are made, to determine the correct volume setting for each speaker.
When the preamp drives the ProAc's selected to that input, you are feeding them the AV signal. Selecting another input (CD, LP) will provide regular two channel listening without having to move any wires.
