Inexpensive sub for MG12's

I have been listening to my new MG12's with a 15 year old Acoustic Research sub that is a 12 inch with 175RMS amp.

The sub sounds a liitle bit boomy, there is no deinition or timbre to the bass, it cannot be adjusted to a 45 hz cutoff, and I suspect that sub is a little bit slow.

What alternatives are out there for about $500? That is my current budget given the WAF is very low over the Maggies (size along with sittng on 14 inch high wood cubes about four feet from the walls). The new Denon 103R and a SUT did not help either I suppose!

I want to make a large step up sonically and if $500 cannot do it, I want to know that so I can decide to wait until stereo spending would be more acceptable to my better half.

Advice is appreciated!