Hi Annunaki,
There are several sizes and types of SVS subs. First, all SVS subs are ported. The PCi, PC+ and PC Ultra as well as the CS (no built in amp) are all tube towers. The others are box subs.
Let me decode some sizing as well. With the tower subs, the first number is the stock low-end tuning. The second number is the height. They also make box subs ranging from 1, 10" to 4, 12" drivers.
Port blockers are also included to drop the low end. For example, with one port blocker in place, a PCi or PC+ 20-39 tunes to 16hz and with two, 12hz. There is a price to be paid in output vs. extension though. They have graphs on the website explaining this.
The 20-39 PCi mentioned in the email is 39" tall and 16" in diameter.
The best way to determine what you will need, is to contact the company with the dimensions of your room, and some info about your normal listening habits. They will not oversell you.
In fact, in my first email to the company, I suggested I was interested in one model and they assured me a model costing approximately 1/2 as much was plenty. It was.
Here's the URL:
www.svsubwoofers.com
Here's the email:
techsupport@svsubwoofers.com
If you email them tonight, you may have a response by the morning.