As I mentioned early on, I'm quite interested in this as well.
I had checked with GPA a couple months ago about the 9.0 CF tuned to 45.5 Hz or the 6.0 CF tuned to 53 Hz, and Bill told me that yes, the bigger cabinet plays a bit lower, but in his mind it wasn't worth the extra size the cabinets take up. I decided that I would try the 6 CF cabs and if I wasn't happy with the low end, I'd throw a sub in there.