Have you had a chance to listen to the B&C's yet Ant? I pondered between a pair of B&C (not those ones) and the Eminence but went with the Eminence as it seemed to have the advantage in the midrange performance which gave me more design options.

How well do you think those B&C's work up to about 300Hz / 400Hz?

Also why are you going for a double wall on the sub rather than just thicker MDF? Are you not tempted to try other materials that don't need to be so thick? Aluminium / honey comb aluminium for example? Maybe form the main body from concrete with lots of PVA mixed in then skin with MDF when nice and dry. I even wonder about thick Nomex as a non-resonant cabinet.