Had a bit of an epiphany tonight.
I had been running my 212 cab with the top 1/2 of the back OFF (it's a convertible cab), so for kicks, I sealed it back up/closed-back. BIG difference all around.
I now believe the modular system was designed with that kind of cab in mind. Makes sense as that is how I ran my TM212X cab when I had it.
I was able to dial in the VX closer to where I want it now, but really need to look at retubing the power section.