The HSU has the ability to go the lowest. The 15" on the Velodyne is enticing. There is no replacement for displacement.
Hard to say w/o doing a comparison though. All things being equal however, you should be get more for your buck @ HSU or SVS since they are an ID company (internet direct). That or they keep more profit. One of the two

.
I don't think you can go 'wrong' with any of the subs. I would most likely pick the HSU since it is designed to run at either 18Hz bass extension or 25Hz w/o. I am not sure on the SVS or Velodyne. Are either of them ported and can you get a port plug if they are?