Which sub should I get?

The hgs15 is not similar to the DD-15.

I have had a velodyne hgs15 which is similar to the dd15 without the room correction, and I now have a jl audio f113. I like the jl sub more personally but they are both great subs. Hard to say one is hands down better than the other one. It would be best if you could take a listen to both because it really comes down to your taste in subs

I had a Veloydne DD-15 and it is a great sub that can be dialed in given the sub's room correction software (manually or automatically). It was a little much for music in my room but great for HT. I sold it and replaced it with two DD-12 subs and I like this set-up better. Faster and more balanced throughout my large room.

I have no reservation in recommending the DD-12.

It would be nice if someone did an accurately measured shoot out between some of the more prominent subwoofers. When one of the audio publication wrote a very positive review of the JL they used a Velodyne to adjust and graph the JL.

I have a DD18 and a truly difficult two channel room. While it was easy to get the sub to sound good at the listening position it would overload other parts of the room. I ended up mounting the sub on a dolly and running the program in five different possible locations. In every case the sub excited a node somewhere in the room. In the end I was able to confine the node to a far corner and place a trap in that area. I don't think I would of been able to do this so accurately in such a short amount of time with any other sub.

I ran the Velodyne program on the 15" Earthquake in the HT room. I slightly elevated its volume then let the HT processor further adjust the sub using its room correction program. I conceder room correction in HT a must have.

The JL and the Velodyne are both fine subs and I'd guess most would say the Jl edges out the Velodyne sonically but the Velodyne's setup program is very good.

IMO I have found REL bottom firing subwoofers to be over rated. They've since redesigned there product hopefully for the better.