Placement, Calibration, Treatment, and Eq, then Multiples
Personally, I think the way to go is:
1) Proper Placement
2) Proper Calibration (a given really)
3a) Room Treatments
3b) EQ (auto or manual)
5) Mulitple subs
I have to say, judging by ear, my HK 435's auto eq does a great job of smoothing my bass response.