I've had good results with the Bugtussel Tegmentum sub (which I sell). The transmission-line Tegmentum has low coloration with good transient response (very low overhang), as well as an extremely flexible crossover/equalizer built in, which is especially helpful in getting a good blend with electrostats.
You might be able to purchase a pair of Eros bass cabinets from InnerSound. Once again they're transmission lines, for low coloration and good transient response. As I guess you can see, I think that transmission lines are an especially good match with dipoles.
Gradient makes a dipole sub which would probably be an even better match, but almost certainly doesn't go as deep as the big Tegmentum does.
Best of luck in your quest,
Duke
