Subwoofers - Front Firing or Down Firing - Which Sounds Best?

I'm totally happy with what my single B&W sub is doing. I only wish it would go lower. Technically, it is producing sound in the lower, 20-ish, frequencies but probably down 5db or more. Its amp+cone just can't pump out the gusto. The 200 watt ASW-650 is nominally a twelve inch cone. Actual measurement is more like 10 1/2 - 11 inches. That's kind of meager by today's standards. I'd like something (or two) that will occasionally rattle my windows. That's all.  

Interesting. Any more opinions?

I'm pretty sure a single 15-inch SW will give me what I want. But being retired, cost is a consideration. In my smallish (12 X 13) room, I could use the tube SVS as a plant stand and it doesn't take up much room (as well as being affordable). Thanks to all for the replies! 

It's just me and the sweet spot fuzzy. Going to give the "swarm" a try first.

How does this sound: My B&W "12 inch" ASW-650 plus a spiffy Polk 12 incher recently purchased for HT and the four six-inch woofers in my B&W 804d3s? Is this "swarm" enough? If it works, I could swap the B&W for HT duty and get another Polk (for symmetry) for the main two channel system. I'd be losing the plant stand, however.

@shakira - Nice response! Simple, clear and to-the-point! I like that...

Well I tried my "mini swarm" today. I brought my 12" Polk sub (HT sub) into my music room, hooked it up and (sound of air coming out of balloon) my B&W sub wouldn't turn on. I was doing some electrical work this week (throwing breakers) and thought I blew a fuse. I opened up the B&W and found a wire looking like it was not connected (to the driver) but inspected the fuse anyway (a LOT easier said than done). Anyway, got everything working and Tool's "46 & 2" never sounded so good. Gobs of bass! Will need to optimize things (ordering another Polk) but looks very promising. Don't have room for the "holy grail" of four subs but think the matching Polk's should keep me happy for awhile. Party on Garth!