Thanks for all the help so far. Here's my room right now with no real tuning at all. The only acoustic treatments I have are absorption panels (3" of OC 703) at the first reflection points on the walls. The speakers are Magnepan 3.6Rs. Adding a pair of dual 12 OBs is about $2500. Is it worth it? Is gaining extension down to 15hz or so going to add enough to the music to justify the expense? I listen to jazz, classical, and rock. Talk me into it.
IME it's not really about extension, it's about power. Panel speakers, even large ones, have good, tuneful bass. But visceral, punchy, MEAN bass? Uh, no.
Before the servo OB subs came along, the only way to get powerful bass was with a box. And boxed bass is crazy hard to integrate with a planar main speaker.
The beauty of servo OB subs is you get all the power and punch while not losing any detail or musicality. You truly can have your cake and eat it too.