totalwex:
Am using Double Impacts, upgraded, and with REW creating a convolution file (impulse and filters) and importing this into JRiver have a resulting flat room response (meaning, all electronics and room) from +/- 5db from 16 to 30k. This is without any subs. The room is very large and, admittedly, favors bass response.
+/- 5db might not sound impressive to spec lovers, but as a final, complete result for the system AND room, it is very good and approaching studio standards (typically around +/- 3db).
SOOO, I strongly believe you do not need a sub with the floor standing DIs if you apply a reasonable amount of room correction -- both physical correction (I use wall deadeners behind and between the speakers) and electronic EQ (such as described above).
Corelli's advice above concerning HSU subs is worthwhile. I use these in studio and HT setups and they work splendidly. HSU builds a very fine product at a reasonable cost and they reach down to 16hz with authority. This 16 figure is of interest because 32 foot organ stops go down this far and produce a powerful roar for the lowest "C" on the organ pedalboard.