For sub options; an SVS PB12-NSD or HSU VTF-2 MK3 should do nicely. The MTX probably sounds tubby, flabby, muddy, boomy, pick your adjective. I know you'll lose something on the low end, but how do things sound without the sub? Which mode(s) are you listening to music in? Asking for bass from both the sub and speakers may be causing some nulls. Obtaining the frequency response of your room would help to see what's going on. A couple of members have a Velo SMS-1 for a decent price that might help.