One important detail; the B&C DE250 isn't intended to be used down to 800Hz.
Altec drivers would be one option if that x-over frequency is required for your
chosen woofers. Another would be to build a 7-Pi type design with the 811's
and the Pi midhorn. That arrangement was very satisfying for me, although
I've since built proper 7-Pi's with Wayne's horn and crossover instead.