Perrew,
These comments are for HT only, as per your description of your system.
Does your HTR have DRC (Audyssey or similar)?
If so, the DRC will both optimize the x-over function and provide smooth FR over the entire audible range (which obviously includes all the room induced nastiness in the bass). Therefore, the "smoothing effect" of properly positioned mulitple subwoofers is much less important.
Once you're at this point, forget about smooth FR.
What's (mainly) left is clean output capability. Since I use Rythmik subs, I'll use them as an example. A pair of 12" Rythmik subs provide roughly similar clean output capability as a single 15" Rythmik sub. The 12 inchers are $749 per unit and the 15 inchers are $899 per. If your main focus is clean SPL capability (as, IMHO, it should be in your circumstance), the single 15" unit is a better value. As Bob noted, you could always add a second unit later - for any reason.
This isn't the most sophisticated analysis, but IMHO, it doesn't need to be.
BTW, from my previous post, you should note that, at 150hz, it was easy to localize the source of the signal. If you want to x-over somewhere near this high in frequency (and there are several reasons that you might, given specific equipment in a specific room), then 2 subs wired in stereo is important.
Also, please note my caveat. Under reasonably controlled conditions, I could not hear signal localization once I got a bit under 100hz. Other people may be more sensitive to this. It's easy to extrapolate from our own experience and assume that it's universal, but this may not be so. Hence my ever present YMMV.
Good Luck.
Marty
