The notion of connecting a sub to each speaker channel and thus, having full range speakers at every position sounds like a good idea in theory, but in practice, it creates more problems than it solves. The whole "sub/sat" system came into being specifically because having a full range speaker in every position does not lead to anything close to linear, accurate bass. Instead, you have independent sources of bass, each one having its own interaction with the room. If all the channels happen to be playing the same bass notes at the same time, then fine, you've got multiple subs spread around the room all playing the same thing. But if different channels are playing different bass notes, now you have all kinds of unpredictable interactions with the room and the various sources of bass. Your frequency response is going to be a nightmare and it's going to change as often as there are changes in independent speaker channels.
As to the Rythmik FV15HP vs. the SVS PB13-Ultra, while the Rythmik FV15HP is capable of more sheer SPL output at frequencies above 20Hz vs. the PB13-Ultra, that output also comes with significantly more distortion. If the PB13-Ultra didn't have all of its DSP processing in place to keep the distortion ridiculously low, it could easily crank out higher SPL figures, just at the expense of distortion is all. The Rythmik FV15HP's distortion isn't a problem. It's still below the CEA 2010 thresholds and thus, supposedly not an audible issue. SVS simply went for more aggressive control, which results in extremely low distortion and a more linear response if you are looking at the entire range from 20Hz up to 160Hz (or even a bit higher). It's a design choice is all. Either sub is capable of prodigious output. They're both great. The PB13-Ultra has a few extra bells and whistles in terms of options in its amp (high pass filter, low pass filter, two parametric EQ bands, etc.). The FV15HP's no slouch in features though either - nice low end filter settings to combat potential room gain issues
No "wrong" choice between them. They're far more similar than they are different.
Anywho, back to the topic at hand. If what you want is nice, linear frequency response that is as close to the same at multiple seating positions as possible, then you'll want to get yourself 4 subs and spread them around the room. Two up front and two in back is fine. So is one on each wall - front, back and both sides. All four corners works for 4 subs as well.
The thing with using 4 subs is that - while you certainly get the most even and linear response at multiple seats, and you get that without even needing much in the way of EQ, you actually get lower SPL readings than if you use two subs or even just one. Due to the huge number of interactions between the bass waves, you create that linear response with no huge dips or peaks, but it also decreases what used to be higher SPL output numbers for the frequencies where the room and sub were reinforcing the bass. Basically, take the SPL level where a lone sub delivers a flat frequency response over a reasonable range. With 4 subs, that's the sort of SPL you'll get, but across the entire range. The whole SPL level doesn't really go up, it's just that all the frequencies that were lower or higher get evened out.
For that reason, if you're going to use 4 subs, you want them all to be individually capable of delivering the SPL level that you're after. You go from one sub to two subs and things basically get around 3dB louder with some smoothing of the overall frequency response and a couple of new dips. Go up to four subs and you knock back down those 3dB that you gained more or less, but now the entire frequency range is pretty close to even with no dips or peaks. So whatever a lone sub was delivering in terms of SPL (minus peaks due to room resonance), you can more or less expect that same SPL with 4 subs - and no more peaks. So if you liked some of the peaks that one sub was delivering, say goodbye to those and realize that if you want those same levels that the room peaks were delivering, each of the 4 subs is going to have to be able to output that level on its own!
So I take it you've got about $4000 to spend and you want some VERY loud output. Two FV15HP is the way to go out of that list if that's what you're after. Putting them both up front, you're gonna need a fair amount of EQ though, so factor that into the budget. Two PB13-Ultra is going to be very, very close behind (and have the lower distortion and greater 20Hz output that we've mentioned), plus you'll have some EQ built into the PB13-Ultra subs.
Four LFM-1 EX subs is going to be significantly quieter - on the order of about 10dB since the LFM-1 EX isn't capable of the same high output to begin with. Stacking two more up front isn't going to do much unless you want to throw your frequency response out of whack again. Stacking two front and two back will boost things up around 6dB though.
Regardless, all of your subs should be acting as one sub in essence. The idea is to use their output to create flatter frequency response, not just create 6 or 7 non-linear speakers.
If what you're after is the sort of SPL that dual PB13-Ultra or dual FV15HP can deliver, then go with either of those. Putting two more subs at the back of your room will bring your SPL DOWN, not up. You'll get flatter frequency response throughout the room, but LOWER SPL. So since it looks like you're going for very high output, go with the dual high output subs and simply factor in that you'll need additional EQ to make the response linear.
