I ordered a single servo subwoofer driver and a A370PE amp (I figured I'd start with one and buy a second driver and amp after next payday) and built two single-driver H-frames (thinking I might as well make two at a time if I'm going to use two eventually).
My big "barrel" OBs with Lii F15s looked ridiculous atop the H-frames, so I decided to modify them and make them more streamlined. Since I wasn't worried about keeping the baffle a certain size for bass extension, I came up with this new configuration:
It's a bit odd looking at an empty H-frame but here's the deal: the bass even with a single sub driver sounds great. Granted, it took me a couple of days of tweaking and measuring SPLs, but the result is everything I could have hoped for: virtually linear (within 2 or so dBs) bass response down to 32Hz. I'm only 6 dBs down at 28Hz. It's amazing to hear such low bass in my small room.
And the transition from the F15s to the servo sub sounds seamless. My head knows that frequencies below 80Hz are handled by the servo sub and those above are handled by the F15s, but my ears just hear music.
So here's the thing: I don't think I need any more bass than the one servo woofer is giving me. On the other hand, I've got the second H-frame all ready for a second sub. It also means buying a second A370PEQ, so I'm wondering if it's worth it.
Given my small listening room (mine is 12' x 13' x 9'), a single sub seems to suffice. So what, if any, would be the advantages of adding a second one? I might be talked into adding a second if there's a strong argument in favor of doing so.
But in addition to this question, I'd also like to commend Danny and Rythmik Audio for making such a great subwoofer kit. It really is as great as so many of you claimed on this forum.