The AudioQue HD's are similar but not the same. The Audioque motor is different than the motor found on the DD subwoofers. They have comparable motor strengths, and they use the same magnent slugs, but they are a completely different design, other than that.
Also, Audioque doesn't have near as many voice coil configurations as DD. You can only get the Audioque's in dual 1.5's, dual 2's, or dual 4's. No exceptions. If you can't make that work, Audioque will tell you to call DD. I think their more equivilant to the 3500 series but a tad bit better but the DD's can be better b/c they can take alot more power than whats rated. Hope this helps ya!
Also, I am pretty sure that Audioque only handles U.S. domestic orders. So if you are not from the U.S., I am pretty sure you have to stick with the DD's. You might check with AQ, if you are really interested, but that is what I was told.