JS, if you haven't made one already, I'd seriously consider Linkwitz Transform (there's an excellent article on it at ESP).
Bag End use it in their hifi subs actually.
Basically, you electrically modify the driver's parameters - you lower Fs at a cost of efficiency. It's a good compromise in hifi when you're using a sealed sub and one that I'm looking into for some sealed or 4th order BP subs for a pair of LS3/5As.
\'bits of ply round a driver\'