Nothing small. The Velodyne will probably be about as good as you'll get. Not sure how big the Rel's are at the same price point, a possible alternative. There is the Sunfire D series which use very small cabinets, but maybe a little more expensive.
The Velodyne may be amazing, i don't know, i've never heard one, although i may be hearing some shortly. No doubt it's a good bit of kit, but i think you'll be struggling to find a small sub to reach 26Hz without spending a lot more money