schlagdog
Inspired
Have you tried plugging your Axe-Fx into the effects return of your 5150, then turning the Axe's Sag control to 0 to disable the poweramp? In other words, use your 5150 as a poweramp only, and the Axe as a preamp section only.
A good way to figure out exactly where discrepancies are is to narrow down as many variables as possible. This might help that.
Also, I wonder how much variance there is between 1 5150 and the next. If you took two 5150s off the line at Peavey and A/B'd them, how different would they sound with the same settings?
I did that also to try and rule stuff out but its way worse doing that. The axefx is so full of honky midrange that they aren't remotely close to each other. You really have to start going crazy tweaking everything just using its preamp section to drive the 5150. The Peavey's don't sound different like Marshall do coming off the line. They pretty much sound all the same.