well that amp isnt giving the subs enough power. youre only giving each sub 250watts. theyre rated for 400 and can handle even more than that. I havent heard the 15" momo, but the 12" sound great. Im sure if theyre given the proper power range they will sound much better. In either case, the upgrade to the sr124 sounds pretty good also. Im doing the same thing
if you can get a hold of a mono amp that gives between 700-1000 watts at 2 ohms, that would work well with those two subs. If there are any generous car audio shops in your area, they might let you test the subs on one of their demo amps and u can see if it sounds any different. if it does, then you know what the problem is. Two of these subs should provide plenty of bass.