Spiky

RXV is the upper line, HTR is the lower line. Usually about 2-4 of them cross over at the low end of the RXV and the high end of HTR. They would be almost if not identical in functionality and quality. I don't know model numbers for the current lineup, but you can pretty much line them up and see which match.

They do this so they can sell the same receiver at Best Buy and high-end shops without having pricing wars. Both places can claim, "We'll beat any price," because the other sells a different model number.

regent

SvS

svadas

Since cost is an issue, I would recommend a used original series HSU VTF-2 from ebay or audiogon. I seem them a lot and for cheap. This is a real sub and not a toy like most of the cheaper models. The downside is that the powder coating exterior isn't the most attractive. It does 25 hertz though and will shake the hell out of a mid size room.

buckee1

Here's another nod for SVS or Velodyne. Both of them are awesome subs. I'll be upgrading to an SVS in the coming year.