Speaker "whistle in "balanced" mode.

You are so welcome. I only wish all problems with audio and life could be so simple. I hope you continue to enjoy your system.

Px25: Swapped out the tubes, and whistle now gone. Thanks so much for your time, and advice.

Probably the early stages of one of the tubes in the preamp going bad. Since the whistling only happens in balanced mode, try switching the tubes in the balanced part of the circuit and see if the whistling changes channels. From what you are describing, this is the most likely problem. If the whistling does swap channels with the tube change, you can change a single tube or for best results, replace all tubes in the balanced section of the circuit.