Re: F12 vs L12?
These are all high-quality subs and it's hard to go wrong here. Rythmik, SVS, and Hsu are generally considered the class leaders in price/performance in this range. There are certainly differences here but they aren't huge, and all three of these are good options.
Relative to the L12, the F12 will play lower and louder, and is more tweakable. A lot of the differences between subs from the same manufacturer are primarily in their ability to play lower and louder, hence the price difference.
Relative to the SVS, the Rythmiks will play lower (particularly the F12), but the SVS will play louder. The servo system in the Rythmiks generally result in a tighter, more defined sound. This is sometimes mistaken for having less impact; in reality, it's just a less boomy sound. Some people prefer the somewhat more boomy sound, so YMMV. Also, the Rythmiks have a steeper crossover available, which may make integration easier but again, this may not make much of a difference in your setup.
Luna Duo V2 LR, Titan Horizon V2, and Rythmik L22 & L12 in HT1; Sierra-LXs in study; S-2EXs and Duo V2 C in bedroom; S-1 NrTs in dining room; S-1s at work; HTM-200s in kitchen. Brother owns CMT-340s and dad has a pair of CBM-170s.