I've always had my eyes open for the Deep Peavey 2x15, the shallower ones pop up with some regularity but I've yet to see a 215D.
I used to use a SVT3 Pro, I dont want to be condescending at all, but you do know that the clipping light is like fully constant red at normal playing volume? I dont know why Ampeg even put it in there, it just scares people from turning their amp up thinking they'll overheat it... I was like that for awhile until enough people told me an SVT3Pro can play happily with everything dimed (even though it'd sound like crap.)
So to coax the most juice out of that sucker that I could, I wouldn't use the graphical EQ, I'd put the Master volume at maximum and control the actual volume with the Gain knob. Usually Master would be maxed and Gain turned up to around 3/4. Turn the "tube drive"/"compression" or whatever mostly clockwise to get more clarity and cut, this is more clean/solid state tone but you won't mush out as easily. Don't engage Ultra High or Ultra Low or the Input Pad or Bass Boost or any of that crap... just crank your low mids like you are now. Any effect that amplifies your input volume will help get you some more headroom too, the SVT3 pre can take alot of juice it seems without clipping out.
Also, you would not believe the difference between a single 2x15 and two 2x15s. When my guitarist was using a halfstack, I could "get by" on a single 2x15. Now that he has the fullstack, I always run a 2x15 with another big 4 ohm load next to it... but you'd have to bump up from the SVT3 if you want to get down to 2 ohms.
Laz spent awhile coaxing more volume out of a 3Pro, may give you some tips. I think he rocked the graphic EQ and maxed the sliders or something.