The Ultimate Subwoofer Co, set up by Gecko in the UK, are selling modified 18" SP4s in an small-ish cube, running a coil off each channel of a iNuke NU6000DSP, IIRC. They have only got a holding page on their website at the moment but there are a couple of threads covering them (and the other USC offerings) on AVForum.
The response on AVForum has been very positive, I've not heard it yet myself but will hopefully do so in the near future
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