m-a, I didn’t mention it, but the Rythmik A370 plate amp (the amp that is included in the GR Research/Rythmik OB/Dipole Sub kit includes a continuously-variable Phase/Delay control, providing phase rotation from 0 degrees to 180 degrees (0-16 ms). That phase rotation creates a time delay exactly the same as physically moving the sub further from the wall!
In fact, that phase control is found on all the Rythmik plate amps, including those installed in sealed and ported models. I would not buy or own a sub without it. While I myself like the GRR/Rythmik OB/Dipole Sub, a few found it to sound too unlike what they think a sub should sound like ("thicker", "plumper", with more "weight") . One who feels that way is the owner/designer of Rythmik, Brian Ding! For those people, Rythmik offers many sealed and ported subs, including the F12 (a single 12" driver in a 1.5cu.ft. enclosure with a 600w amp) and F15 (a 15" in a 2cu.ft. box, same amp).
Lots of info---including unusually technical---available on the Rythmik website. For discussions about the OB/Dipole sub, take a look in the GR Research section of the AudioCircle Forums.