The BW/OCT is equivilent to Q, or the width of the filter. When you use a 20hz subsonic filter it cuts the low side of the slope and the resulting curve looks very close to the BX63 shape that way.
Thats what I use with my B380 clones and the 4645Cs. Allthough as you have pointed out, room gain does re-enforce it most places that are as small as the typical home listening room.
