hi andrew
off the top of my head you may struggle to find a 15" driver that will fit your needs(at a good price to performance ratio),most with a decent freq bandwidth that goes up to meet another driver usually need a ported enclosure,even then i doubt the low end response will be that low within a half sensible enclosure volume(i thought of building something like this,a diy version would certainly work out a lot cheaperhttps://positive-feedback.com/review...-loudspeakers/ ).
of course i havent looked at every driver out there,just a few when i was researching for some ideas,as for a 200litre cabinet...
feel free to stick up your ideas and plans here if you like,always interested in that kind of stuff...probably why im so slow at building anything,to busy reading about it
im of the same view as you regarding transmission line bass quality,i do like a nice transient response that a sealed enclosure brings to the party,hence my desire for a sealed design,although it has to be said that a well designed ported design can be no slouch these days.
also like you i would love some of those mega expensive jbls...if only