have a Peavey Classic 50 with the EL84s etc, and i know whistling from an amp is a sign of a tube starting to kick the bucket, but at high volume with the crunch channel i start getting a reverberated whistling sound, its not very present but on the harder notes i can hear it. now i didnt have it cranked max, but the peavey was at around 5 on master volume.

just trying to find out of microphony from tubes is always a sign of a bad tube.