I had an RW-12D and a Sub-12 and recently purchased a used HSU VTF-3 MK2. I hooked it up, disconnected the Sub-12 and moved the RW-12D to the position where the Sub-12 was located. The difference was immediately noticeable. The HSU produces a lot more bass and is much cleaner and tighter. After listening for a while, I had to disconnect the RW-12D also as it was degrading the overall sound quality.
The current version of the sub I bought is the VTF-3 MK3. The VTF-15H would be a huge upgrade over the RW-12D, but you don't need to go that big or spend that much. I bought my used HSU for $400. The guys at HSU are very helpful. Let them know the size of your listening area and your budget and they'll help you make a good choice.
I thought the Klipsch sub combo I had sounded pretty good, but after listening to the HSU, there is no comparison.
