Two SVS subwoofers.
I picked up a PC13 Ultra on Friday to go with my PC12 Ultra. I wasn't unhappy with the PC12 Ultra, but I kept thinking that a second woofer would help to smooth the response a bit. Apart from the fact that the PC13 is about 6 inches taller, thus destroying the symmetry of the set up, I am quite happy with the new addition. BTW, main speakers are Martin-Logan Vista's with a Stage center channel.
Per recommendation from Ed Mullen, I set the volume on each sub individually (using the AIX blue ray sampler test tones), with the PC12 Ultra about 3db lower in volume than the 13Ultra. I then ran Audessey on my Marantz AV-8003, and left it at that. As expected the addition of the second woofer decreased the subwoofer setting about 6db on the Marantz.
There is a difference in sound quality with the addition of the second sub. How much of this is due to the difference between the 13 ultra and the 12 ultra, I can't be totally sure. I did set the 13 ultra where I had the 12, and ran a couple of CD tracks before hooking both subs up, and it did appear to me that the 13 sound added a bit more texture in the bass than what I heard with the 12 ultra, but it was a short listen.
With both subs opperating, I definitely hear a better defined, and I believe smoother bass response. The bass in general now seems to be more a part of the music, rather than a separate entity. The bass appears to be more effortless, like it just happens, as opposed to being generated by a speaker. Stand up bass now seems just a bit more life like and bass drum and tympany strikes bloom a bit more than they did before. All this while at the same time reducing the bass node that I used to find near the little alcove where my turntable is set up (good thing).
We've only watched two movies so far (League of Extarordinary Gentleman, and Sherlock Holmes) and the improvement is noticable. The combined subs seem to go deeper, (both with all ports open) and with less effort. In Sherlock Holmes, there was some really deep stuff that happened when Holmes was visiting Blackwood in prison that made me and my wife turn to each other and go "wow". Same thing with some of the scenes in the League (which we had watched a few times before with just the 12 Ultra).
At the same time, even though the really deep bass seems stronger, there is a bit less boom, and the whole sound field seems more integrated. Gunshots have a bit more impact (We all jumped in seats during one part of Sherlock) doors closing just a bit more weight, but without overhang. All this at levels that are ~ 8db below "reference".
All in all, a satisfactory up grade, and if anyone is waffling about the merits of adding a second sub, my experience is that it is a worthwhile upgrade.