As usual, I largely agree with what Bob R has said. He represents one major school of thought, Stan represents the other. However, you eventually have to pick your own school.
You can really add 3 major benefits by going to a sub or multiple subs:
1) Deeper and/or louder & cleaner bass response - because a sub is specifically designed for the task.
2) Smoother bass response via optimized placement of the bass generator. The subwoofer can be moved closer to the wall reducing certain cancellation effects, especially with multi-sub placement. Bass specific Digital Room Correction devices also allow for smoother bass by EQ of the bass signal headed to the subs.
3) Improved performance of your main speakers by relieving them of bass requirements. This is probably quite applicable to your B&Ws.
The "cost" of these benefits is trying for perfect integration and minimizing "intrusion" into the signal path.
Stan's approach gets (partially) benefits #1 and #2, but some argue that it is the only way to minimize the "cost" of adding a sub. The signal path can stay "pure" with no new devices between the signal and your speakers.
Bob's approach goes full-out for all 3 benefits, but you've got to actively cross your mains, which means an active device between your pre and power amp. Bob also favors subs (SVS) which maximize deep clean output capability while Stan favors subs (REL) that are designed to minimize bass bloat at the expense of extension and output capability.
Pick your poison - then pick your subwoofer.
Marty
I should add one or two more things. I've heard a lot of RELs, but never SVS, so this observation is from test data only (FWIW):
The less expensive sealed 12" SVS subs actually test quite well for reduced "bloat" (though not up to REL standards) while the less expensive RELs have so little extension/output that they are really woofers, rather than subwoofers. For music, some would say that that's a distinction without a difference. Others would disagree.
I use two 12" Rythmiks which are somewhere in between.
Also, I own a pair of Verity Parsifal/Encores. The Parsifal monitor is spec'd to 55hz (not too far from your 805s). When you mate the monitors to the Encore woofer cabinets, the internal passive x-over occurs at 150hz. When I asked Verity why they chose to do this, they pointed out the "dynamic" benefits of lifting the load off the monitors, even though they can go much lower.
If you choose to cross subs anywhere near this high actively, you'd need to run 2 subs in stereo. In this case you'd also have the benefit of addressing the room issues between app 100hz and 125hz (in my room), which I find most annoying of all. You can address this range with placement and/or DRC of your subs and/or bassbusters (which would allow a lower x-over point).
Just FYI.
