Grissom has said several times in videos that he usually runs his guitar volume around 5-6 to get a nice crunch tone. Then he can control the gain of the amp from the guitar, and get a lot more gain if needed, or clean things up by rolling down the volume.
He also blends the pickups a lot, hence the two volume controls.
Don't know if you've tried this, but no doubt the pickups are designed to work best for Grissom's techniques and tones, since he picked 'em.
I actually do the same thing when setting up my amps, so I understand why it's done this way, and why the pickups are the way they are, though I don't have a Grissom model - yet.
I'm always curious why folks buy a guitar with pickups they don't care for - with respect, not arguing with you, what was it about the DGT that made you pull the trigger?