your best bet is to use the Xover within the B15. It's quite nice and matched specifically for that speaker. Get a Sound Level Meter from Rat Shack and use the CD that comes with the Revel to set up the parametric EQ in the sub. Integration doesnt' get much better than that. I have a Bryston 10B pro Sub X-over and it's a nice piece, but if I had a Revel b15 I wouldn't bother with it.
-Ed
-Ed