Does the Grissom amp just give another variation of the "classic Marshall" thing?
Nope. I have both amps, and they're quite different.
The DG30 isn't based on a single amp - instead, it's a blend of attributes that David Grissom liked about his vintage Tweed, Vox AC30, Hiwatt, and vintage Marshall amps. It's kind of a 'greatest hits' amp, if that makes sense. While it can be driven into gain and kinda-sorta go Marshall-ish, it's different from the HXDA.
To get a good idea of how the amp sounds in a pretty crunchy mode (Grissom plays pretty crunchy), it's all over Grissom's album, 'How It Feels To Fly' that came out a couple of years ago. Listen to the title song, and you'll hear how the amp has a different voice from the HXDA.
I don't remember much, if any, clean tones on the record, though that's probably just my memory. However, the cleans are also very different from either Marshall or Fender cleans. They have more midrange, the pick attack is very fast, and it's altogether different from any of my other amps.
So it's kinda Tweed, but it doesn't fart out on the low end like an old Tweed amp; it's kinda Vox, complete with a cut control, but the low end is deep, unlike an old AC30; It's responsive to pick attack and has a rich midrange like an old Hiwatt; and the mids and overdrive tones are different from an old Marshall.
It also dials in differently, because the Master volume is pre-phase inverter. so the volume (gain), tone and Master are interactive. This makes it a little bit trickier to dial in, but once you get the hang of it, you're in business.
I would say it occupies its own tone world. I love both the HXDA and DG30, and am glad I got one of each.
From time to time, I've been an inveterate amp flipper, but I bought both of these amps in 2014 and wouldn't consider selling either one.