@peter_s - it was the X4 you heard at PAC. That's where I heard them as well. They were pretty impressive, particularly for the price (~8K).
Don and Jay from Reference North Audio built my Line Force cabinets. They make a lot of the flat packs for GR-Research products (NX-Oticas, NX-tremes, servo subs). The Line Force cabinets are made from composite in order to be strong and stiff enough with all the driver cutouts, so they take longer to make and are considerably more expensive than the MDF flat packs. As far as I know, they are happy to make more of these cabinets.
GR-Research has NEO3 drivers available, but the Bohlender-Graebener NEO10 drivers are no longer available. I purchased my BG Neo10 drivers more than two years ago when they were still sporadically available.
Danny from GR has been trying to make a suitable replacement driver, but so far he hasn't been satisfied with the prototypes. Parts Express has their GRS 10" planar driver available, which can be used with suitable crossover changes, but this driver drops off quickly in the lower frequencies. Danny is working on a crossover using this driver, but I would expect he will have to compromise efficiency to get a flat response. They will probably sound great, but will need more power to drive them.
This speaker is not inexpensive to build, even though you are only paying for the raw materials. I spent well into five figures for all the drivers, crossover parts, wire and enclosures. And this doesn't include the pair of open-baffle subs that are required since the Line Forces only play down to about 170Hz by themselves.
But they are pretty stunning, even without being broken in. I've heard speakers that rival these, but not many and all well outside of my budget.