AG500 with DB112s is going to be really strong in the midrange. Just be sure you're looking for very focussed mid-punch if you go that way. I would see the AG as being better with the GS112s, and the 750 better with the DBs.
+1
what he said, and then some.
the DB112's are ok for kicking thru that super dense mix of 10 keyboards onstage, but frankly, that ballpeen hammer like midrange would seem like it'd get fatiguing after a few sets.
the GS112's a great cab, and the EQ on the AG500 is nicely matched to it. extra kick to punch thru? hit the low mids knob. need more definition? hit the high mids. too honky in the the mids, pull back on either. need to get a fatter sound in general? crank that lows knob. again, the EQ is magic pants w/ the GS112.