I've never encountered that problem, but that sure as hell ain't right.
Is there any difference if you use the effects send instead of the main outs (or vice versa depending on how you have it connected to your power amp)? You should be able to get a massive lead tone with the volume on 2 or 3 and the lead drive around 7 or 8.
Tried the main outs, the effects send and recording out. The recording output takes away most of the high end sizzle, but then it sounds like I have a thick blanket over the amp.
I should have mentioned this in the OP but I've tried using a 6505+ and a Marshall 3203 as power amps (both into a V30 cab) both of which give the same poor results. Even straight into my computer with impulses it doesn't sound quite right.
I've seen all of those YouTube demos where people are getting massive lead tones by plugging straight into the amp with Lead Drive on like 5, so something definitely seems off with mine.