I would recommend the Sunfire Signature subwoofer.
Contrary to the negative propaganda, properly configured Sunfire Signature subs are very musical. I am using a stereo pair with MG 3.5/R with excellent results. The blend is seamless. The bass is fast, smooth and effortless, defined and tight (no trace of "boominess")
For best results, you should try to keep the Sunfire away from room boundaries (contrary to sunfire's recommendation, the sub has too much output and can sound "boomy" when placed in corners). Try to be easy on the volume control (keep as low as possible) and use the Phase adjustment for proper blending.