Px25: Swapped out the tubes, and whistle now gone. Thanks so much for your time, and advice.
Speaker "whistle in "balanced" mode.
Have a solid state amp, tubed pre, and solid state one box player. Of late have been noticing an occasional high pitched whistling sound with amp switch in balanced mode. Only happening with one channel. Sound disappears when switched to single ended mode. Am currently using balanced ICS between amp and pre. Any ideas/advice appreciated.