My $.02. There is no point in using such an expensive driver for this type of sub. Its like going off road with a lamborgini. A band pass sub, which Dr. Geddes is a big fan of, and has seen some resurgence lately, is an acoustic filtering device limiting the sub's bandwidth and using this to boost the efficiency in the pass band.
Usually, the difference between a good woofer and a great woofer is the bandwidth you can use the great woofer is greater than the good woofer. If the design you are using is a bandwidth limiting, then why waste money on a great woofer when a cheaper woofer is likely to suit the task, probably even better?
The bandpass sub seems like a good idea, though I've never heard one outside of car audio. You can find tons of information about them on the internet. Decware has a bunch of bandpass sub designs on its website. Like I said, Dr. Geddes uses 6th order and 5th order bandpass subs. There are quite a number of discussions on bandpass subs in the diyaudio.com sub forum.
One of the things that seems common is that they do not often use expensive drivers, they use drivers that have the right properties for the job which is usually reasonably priced drivers. You don't need tons of xmax because you are limiting the low freq content and you don't need a super motor or exotic cone because you are limiting the upside as well. It just has to operate in a limited range really well, which all half-way decent drivers do.
Lots of folks have been using the Tang Band W8-740, which is an 8" driver almost ideal for the task and costs $60. I am itching to try it out myself.
Another point. Port noise seems to be a common issue in some bandpass designs, so care is taken with respect to size of port and roundovers of the port openings. This would make me leary of using a slot.
