do you have the ability to vary the location of the sub? If you can move it 8-9 feet closer to the listening position than the mains, or time delay the mains, the timing / phase relationship can be carefully optimized. Once that's dialed in I think you'll be startled by how seamlessly the sub integrates with the mains.
absolutely terrific write up here:
http://www.soundoctor.com/whitepapers/subs.htm
absolutely terrific write up here:
http://www.soundoctor.com/whitepapers/subs.htm