Hey nomorepeole (I like that BTW..). don't fret, what you want to do is "exactly" what they made it to do
. Yes tube poweramps should "normally" have a load (a speaker or resistor or a load). the MP2 is, as you say a preamp. It doesn't need a "load" in this context. The cab sim outs (XLR or 1/4" jack) can be plugged directly into your desk (if XLR (which is good balanced, low noise) or 1/4" jack > channel line in, both will work fine. If you use XLR (which is my preference), then you need to adjust mixer input gain (selectable on cab sim outs, use mic not line lv for XLRs). If you take the 2 1/4" jacks to 2 line ins, then select line lv on the ADA (which is the way I plug in my MP2 every Friday night.... MP2 > Behringer Eurorack > Behringer Truths... and TC MoneXL on the stereo FX send/return).
Turn off your power amp, in this context you don't need it. Take either the XLR or 1/4" cab sim outs to your desk (2 channels, pan them hard L R), adjust ADA cab sim out for correct gain (mic or line, depends what you went for, (BTW 1/4" jack are the easy way then plug into and select line in). Then connect your GMaj to 2 FX sends (1 & 2) and return it down 2 more channels (panned hard left right), heaven on a stick