Wow that is funtastic result for such a simple and inexpensive subwoofer config. Where do you cross those subs? Did you ever noticed "sounds coming from the back"? How much SPL at listening position can you produce with those two subs (visaton drivers in H frames)?
This FR is impressive, but remember SPL and low frequency response will be generated by cone area, cone movement and cone resonance. Now nothing is disclosed about the speaker units, so SPL dB is not known and 12" as judged what markus said himself is probably a SLS12 from Peerless. By itself it won't play excessivly high in a Ripole. But be a superb SUB. As Ripoles in fact are exellent OB SUBs with the right speakers.