26Hz is low enough for an AV system, but i'd be a little dubious about any sub reaching that using only an 8" driver, not at any sort of volume anyway. You'd need to be looking at 12" minimum.
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
This will go that low but it'll bust your budget a bit. I am assuming you want to stick with a compact sub though.
The D8 I heard, had prodigious volume capabilities for it's size but once it hit about mid 30Hz (ish) everything just became one note with no real extension on offer.
It was worth it just to see the cone travel. If I were stupid rich, I'd buy one and put it on my desk as an executive toy.