I have a Vac amp, which does have a high gain. I believe
38, as opposed to many other tube amps, which are closer
to 30. I only open my V-C to around 9:00 ot 1/4 turn,
which is loud. The manufacturer of my preamp said not to
worry about this because the type of volume control he uses
in his preamps do not change in sound no matter what level
you are playing. FWIW, I also heard a VAC Ren sig 2 on a
VAC amp, and I believe this sound was also loud at around
1/4 turm. My pramp is not a VAC, but does have 20 db of gain, the VAC's I know are higher gain as well. Not sure of the spec. I guess even though VAC amps have the higher
than average gain, Kevin still uses a fair amount of gain in his preamps. When I heard a VAC amp at a dealers recently, he was using the VTL pre. Again, a higher gain pre paired w/ the Vac amps. Regardless, I do feel it would be better to open up the V-C a bit more and/or have smaller step increments on it as well. Just my related experience, hope it helps. It does seem like a synergy issue may be involved though. You may want to call Kevin,
and let him know your amps specs, and then your amp manufacturer, and let him know the preamps specs.