There are various treatment options which are designed to be installed in T-bar drop ceilings. If you get some designed to diffuse sound, they won't deaden the room (like absorbers will), but the at the same time the ceiling won't act as a reflection point.

Also, if you're installing left and right walls, sacrifice a little width in the back, to make the room taper, like a parallelogram when viewed from above. This will go a long way to preventing standing waves. Oh, duh, I didn't think of the perfect example, like Axiom's speaker cabinets. Also the false walls will make it easy to fish wire for the speakers in the rear of the room.

Stiff walls are a key to allowing the room to be pressurized, and accentuating bass notes. When constructing walls, build the stud frame, place glue (like Liquid Nails) on the studs before screwing the drywall to it. After that sets, screw a second layer of drywall over the first with another layer of glue in between. Offset the seams of the drywall panels so the gaps in the first layer are in the middle of the sheets of the second. Finish the second layer with the usual mud (don't worry about finishing the first).

It would be nice if you could construct the ceiling in the same manner, but as long as the floor above the drop ceiling is solid you should still be able to compress the room's airspace--you'll have the subs to do it.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris