Being on somewhat of a budget we bought an Infinity PS-12. It came Friday and we're very happy with it - it can go deep/low, as well as being clean with plenty of power reserve. It is able to shake the entire floor of our house upstairs (where the theater is getting set up). The TJI bracing and insulation packed between the floors/walls/ceiling effectively controls the bass from resonating too much or getting boomy...yet it vibrates the floor and seats nicely.
The sub it replaced was a Sony SA-WM500 subwoofer which was temporarily "borrowed' from our family room to get sound running with our new setup as quickly as possible. If the Sony had been able to take the punishment we might have just stuck with it, but it seemed stressed under high volume heavy LFE scenes (pod race etc.). I was at least curious enough to A/B the two and not surprisingly the Infinity won hands down, though for almost half the money ($150) the Sony is actually pretty acceptable for a low budget sub at moderate volumes.
I could not resist the temptation however, so also had to try running both of them together...MORE BASS!
Under the theory of "move more air" this could have been a good thing? Of course this is not scientific, but IMHO the solo Infinity sounded better on it's own - the two seemed to actually have less lows than one (maybe due to some cancellation issues). At first I tried them stacked on top of one another - easy because the Sony has a Line Out connector on it...but being less than satisfied with the result I also tried a L/R front setup as well. It really just didn't do what I was expecting (more shake factor), in fact probably a bit the opposite. I also experimented with switching the phase around on one - to possibly offset less, but that had no noticeable affect. I might have had more success experimenting with more locations, but we still needed the Sony for our family room anyway and it will do just great there!
Just anecdotal observations - under controlled conditions I am sure more bass could be achieved with two over one, but it clearly would take more tweaking than just plugging them in and powering up, and I was not willing to take the time to work too hard when the Infinity was competent for our needs on it's own. So just a work to the wise...if you're going to attempt multiple subs, be prepared to have a fair amount of work ahead of you to get them working properly together in tandem!
