Been there, done that except made them taller to get ~20 ft^3 cabs tuned to Fs to replace A7s with no regrets, so IMO the empty 825/828's ~15 ft^3 gross Vb tuned to ~24 - 32 Hz depending on amp and room/position variables will have all the dynamics required for all but possibly the largest pipe organ symphonies in a HIFI app and do it at audibly lower overall distortion than if expo horn/BR loaded.
If the budget allows, you can match the A7's overall dynamics (and then some) by adding a second 416B/channel and doubling available power and possibly with a lower tuning or even sealed up depending on the room/position variables. This is what I later did except swapped to dual 515Bs.
http://www.hostboard.com/forums/memb...-altec-sub.jpg
GM