I have yet to understand why anyone with a modeling amp such as an AD120VTX would want to hook it up to a modeling MFX unit.

If you prefer the modeling & FX from an MFX unit such as a TLST or Boss G10, my advice is either put it through the PA or use a straight non-modeling amp.
Arguably a flat response, full range (FRFR) amp will release the true potential of your MFX unit's modeling because it will 'colour' the tone less than a conventional amp and allow the modeling to shine through. But having said that I get superb valve tone using the TLST direct (not in the FX loop) through my Class A Laney VC30-210 & 1x12" extn cab.
But IMHO a modeling MFX unit with a modeling amp is a big 'no-no' simply because the different amp modeling will conflict - unless you turn OFF the modeling and cab models in your MFX and just use it for FX. Either way, it seems to me guys that you're either wasting the potential of your amp, or the modeling MFX unit.
And as for the Boss GT10 (& I have experience with all the GT units - I even owned a GT3 & GT6 for a while) - Yes, it has great effects, with some excellent features, and functionality - but how anyone could prefer the GT10's sterile digital 'COSM' amp modeling to the warm valve-like modeling in an AD120VTX is totally beyond me!
I would add though that the latest Boss GT12 is a big improvement. I tried one recently and have to say that it's the first Boss unit I've played that is definitely 'getting there' & is so far the best modeling I've yet heard from a Boss product. It's still not as organic as Vox, but it's noticeably more responsive, warmer & more natural sounding than any of its predecessors.
Horses for courses I suppose - but just wanted to throw in my 2 cents for whatever its worth.

"I started out with nothing ...and I've still got most of it left!" (Seasick Steve)