Thanks for the help. I e-mailed SVS and communicated with Tom V. Because the space this will need to fill isn't very great, it's a small HT setup being built around my computer, a PCi should do fine. He wrote that until the PCi begins to reach its "clean output capabilities, the two models will sound nearly identical".
I have a feeling I'm going to be very happy with this sub and SVS. My family has a larger room for more serious, collective movie watching, and if I can convince the other members of my family to get a sub for there, I'll probably need a "+" to do it properly. The only problem is the convincing. They don't like bass half as much as I love it.
oh, and I'll probably get a 20-39 instead of a 25-31. It costs only slightly more, and the greater extension might be enjoyable. Plus, Tom recommended it, and I must serve Tom.