Very Compact Subwoofer

I could maybe use a "tube" subwoofer if it could be stood on end/vertically. Again, I'd be looking for it fitting into a less than 13" x 13" footprint.

I see there is also a smaller HSU Research sub; I've got some options now! Thanks, everyone, for the leads!

Leedistad, No, I would not build these subs into a cabinet. I'll be using them standing alongside a pair of Eminent Technology LFT-VI speakers, which are wonderfully open and throw a big soundstage but need some low end augmentation. I would like to get the low end down to about 30 Hz realistically for such a small sub.

An Infraplanar sub would be ideal, but I don't want to spend tripple the entire system cost for a sub. Not going to put that kind of money into a second system.

A follow up for all my helpful friends; I ended up purchasing two HSU STF-1 subs. They are the smallest full range sub HSU makes - 8" driver with 150W powered. They are (going from memory) 14L x 11"W x 19"H, which was the right dimensions for me.

The sound is extremely good, better than I figured for a powered amp 8" driver at $300. The volume and frequency response controls need to be dialed in appropriately or else it gets very sloppy, but the range is very wide and accomodates integration with the Eminent Technology LFT-VI speakers easily.

I'm very pleased with the performance, much better than I expected at the price point and size. As long as they are not overdriven or set up incorrectly they are a good solution to adding respectable bass below 30Hz in smaller environments.