I have a pair of HSU MBM-12's (mid-bass modules) running full range off a "Y" adapter from my SLE output. It's set to about 2/3 volume max volume. I have a REL STORM III connected to the left and right main leads. The REL is set to full range and turned up about 1/3 max volume.
The MBM's add the coveted "slam" that "feels so good" on rhythm and soundtrack parts. The MBM's blend very well with the REL and RF-7's, and the blended Hz/SPL's sound very smooth. The MBM's are not priced too far up there - about $500.00 each.
The Fathom and Ultra 13 are two of the highest rated sub's for their sizes. As noted, the SPL of the ported Ultra may be the most well-rounded choice for both music and HT near your price. I'm sure a Fathom would also be a great choice, too, and about twice the cost.
Danley also has mid-bass units, and a 12 inch sub worth knowing about: http://thesoundbroker.com/Danley.htm