I think there's 2 factors:
1. Velodyne is apparently selling these by the bucketload, so there's bound to be a few showing up.
2. Subs are very difficult to integrate well in a 2 channel setup. With good full range speakers, an improperly placed and/or crossed over sub will do way more harm than good. The DD series MAY prevent sonic disaster with the integral room correction eq, but the directions have to be followed to a T. And then, of course, a really well-integrated sub is noticeable, but subtle, except in certain passages. So there's a lot of potential for disappointment, given the amount of $$$ people are spending on these things. I suspect that some people blame their particular sub and decide to try a different brand.
Velodynes IMHO are as good as subs get, with less than 1% distortion - which is phenomenal. REL is also possibility.
Keep in mind, when choosing an expensive, high-powered active sub with proprietary drivers and circuits, that service may be necessary someday - and you'll want the company to be around then.
And Velodyne's customer service is superb. I have called in and been connected directly to a chief engineer without begging. And I know of a couple of people who have sent old busted ULD's in for repair and had complete upgrades installed at no extra cost.
So I wouldn't worry too much about the classified listings.
