Except I'm not lacking on output for anything now with just two caps. My demo scenes are played nearing reference with the subs 6dB hot or so and the two caps on the XLS-5000 handle that no prob with the proper HPF in place.
My curiosity in this experiment was if lower frequencies matter as much as suggested by some of the long timers on this forum.
I'm on board that down to about 15hz matters...
I'm not convinced yet -- less than 15hz matters...
The sealed sub acquisition was to have some decent time to play back and forth and figure that out.
As such I ordered four of the 18" ultrimax drivers from partsexpress just now. I don't think they'll charge till they ship. If I change my mind in the meantime I can cancel and not be out anything - but I secured my place in line for the first shipment (I hope). Since I've already seen several people state they were ordering some soon, and likely more will follow when the word gets out that these are available.
worst case scenario - I sell them, or use them as nearfield transducer subs right behind my seats and keep the caps up front!