It appears to me that you would benefit more from optimizing what you have now (20Hz-28kHz), and add a sub later if you still felt that it was necessary. Things like dedicated AC mains, cable upgrades (pcs, ics, speaker cable), and possibly stouter amplification, can all have a substantial impact. Not just in the bass, but as someone else commented, across the entire range. I'll third Glen's MIT experience. I just upgraded to MIT Reference speaker cables and ics, and the bass extension and quality was immediately noticeable.