Very Compact Subwoofer

Anyone have an opinion on the performance of this sub:

Anyone know of other ultra-compact subs; must be 13" square or less,powered.

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I believe the ScanSpeak Revelator 23W-4557T ten inch diameter aluminum driver will work in a 1 cubic foot sealed enclosure. Contact Rick Craig of Selah Audio, as he's built subs for me with these drivers that are phenomenal for music.
The Von Schweikert vs-1 is a tad larger than 13". It is a great sub. Easy to set up, solid, phases well with electrostatics as well as regular speakers. Better than Martin Logan dynamo, snell, m&k and comparable to REL. IMHO.
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.