The 25-31 models have the most output in the bass regions you can hear. Add the fact that they're cheaper than their 20-39 and 16-46 cousins and they're easily the bang-for-the-buck champions ... for me at least.
Extension vs. price, I go with the 20-39 Pci. Don't worry about output, there's more than you'll ever need. And I know of no other sub that will go THAT low for the price.
Well, count me as one of the people who thinks a single 20-39PCi has plenty output for my home theater. I have a fairly large living room, probably about 3500 cubic feet, but due to the layout the sub is only about 8 feet from the seating positions. I have the gain on the sub at less than halfway, and that's plenty loud enough for the levels I listen to (5-10db below ref).
My vote would be the 20-39PCi or the upcoming 20-39PC+. I think the 20-39 subs strike the best balance between total output and bass extension, and now that the price difference from the 25-31 is only $50 it's a no-brainer. Getting the 25-31 tuned at 22hz seems kinda pointless to me with the new pricing because you're giving up the SPL advantage and still not getting the full extension of the 20-39.
Difference from what?
The difference between the current PCi and the PC+ will be more or less identical to the difference between the CS and CS+.
Within a series, the 3 models trade size and mid bass output for extension. Whether that extention means anything to you depends on what you are listening too. There are a select few movies (like 3 or 4) that could benefit from the 16-46. The rest would be plenty happy with the 20-39. A larger number could put a strain on a 25-31 if no subsonic filter is present (ie CS no filter vs PC has a filter [better check with Tom on this though, I'm not 100% sure of it]).
For music, unless you are into pipe organ, the 20-39 or 25-31 will do fine.
16-46CS is in purple, 20-39CS is in red, and the 25-31CS is in light blue. Link Removed
This graph will be very similar between the models in all the different series, single or in pairs (CS, CS+, PCi). The difference will be in the overall output ability.
Which series and whether you want a single/pair will depend on the size of your room, where you will place the sub and what kind of output level you want to be able to achieve. Which model in a series will depend on what you will listen too. Send that information to SVS and they will tell you what options they have that will achieve your performance goals.
If you want to get a better understanding on what the tuning frequencies mean (ie how a port works) I suggest your read this essay: Link Removed
If you are on a tight budget go with the 20-39 (any flavor) but if you can afford a few more dollars definitely spring for the 16-46. The added low bass thump is what subs are all about. Most people don't listen at that high of volume to notice the SPL difference BUT you will notice the Hz differences the minute you put a movie in.
TedO
P.S. I have a SVS 16-46cs+ (for sale) and a just finished a Blueprint 1503 Sonosub, so I have experienced low bass and high output. If I had to chose I would pick in favor of the lower output.
I have a 16-46 and I think the extension is worth it. I play my sub at LOW volumes (apartment) and the effect is still awesome. I often rest my feet on the coffee table in front of me and The Haunting had that sucker vibrating even when the bass wasn't very audible. That sort of tactile response is what the 16-46 (and other bottom feeders) are all about.
I just received the 20-39 PCi. I too live in an apartment and try not to play to loud. While watching the director's version of DUNE my wife got startled and said the couch moved. That's base. By the way, if you want 2, you'd better have a really big room, the sub was bigger than I imagined.