"How do you like the Rythmik? Their dual 15s look very interesting as part of a swarm.
It's been long enough now that I can't remember exactly why I chose the 12" over the 15". Certainly, a big part was WAF, but I guess I wanted something light (!?!) enough to relocate periodically. My HT/family and my home office share a wall, so I figured I'd move the sub into my office and move it back for movies. Of course, the reality is I've done this a scant few times.
In any event, I've found that the F12G is adequate for my needs most
of the time. However, I have discovered nulls in my HT room where the low bass/low frequency just disappears into never never land. There's plenty of LF presence in the primary listening position. I definitely feel the need for a second sub (at a minimum). Also, I've discovered that the 12" can be pushed beyond its limits with some music and very demanding movie sound tracks.
If I were starting over, I'd buy a 15" for HT. Or, for a little more money, I'd choose either Duke's SWARM, or a 15" plus the Rythmik Double Eight subs (or GR Research's double sub/speaker stands. The double subs and the speaker stands won't give me that sub-20Hz sound, but they will nicely fill out the ~20 to ~100Hz range.
If my wife and I can figure out what the heck our plans are, I'd buy one of the sub choices above. [We may decide to move, we may decide to go 'vagabond' for a year or two, or we may decide to return to living in Taiwan.] This indecision is driving me mad as they impact my choices for upgrades in the near-term.
I hope you can find a solution to your subwoofer needs.