Forget those DDR subs, I have a Von Schweikert VR-S/3 sub, UK retail would be approx. £2800 when new. I have been using it modestly in my own system since ceasing to be their UK distributor.
For the right price I would let it go. Is super fast and despite it's size will integrate with small stand-mount monitors.
This is NOT a sub for people who only think bass is for big explosions. But, if you want the definition of a kettle drum in and orchestra or just something that is free of wallow and overhang, this could be your thing.
Warranty is fully transferable. I might still have the pdf spec sheet so if you are interested drop me a PM.
I will be away in the States from Oct. 20th to Nov. 5th though.