Good question about why so little space in such a seemingly large room. It comes down to the "multi purpose" part. It's best to consider it a living room that happens to have a media system, and on top of that has a very specific design theme. The subs have to live on the front wall, along with the mains and center. This may not be ideal for acoustics, but that's the location that's allocated. Add a big L-shaped sectional on one side, a very large linear fireplace on the other, and a 14 ft raised bar counter at the back, and there simply isn't any remaining wall space for subs, or places to hide them. Add to that a need for them to (relatively speaking) disappear in the room visually, and small is the name of the game.
The media system is "my part" of what is otherwise a mid-century south-pacific themed room, and even I agree that big subwoofers don't exactly seem compatible with a "breezy island feel", even if exquisitely finished. So the idea is to make things as discrete as possible, while still having enough headroom to occasionally crank up a sci-fi blockbuster.
I agree about our listening levels being low compared to most, but I also agree with the point about needing peak transient headroom, particularly with minimal room reinforcement and bass-sucking concrete. I very much suspect i'll have to run things hot to get the effect i'm looking for. And simply because we listen at more "conversational" levels, the lowest bass often goes missing from the mains because they're not being pushed hard enough to come alive. A sub that can plumb the depths from right off idle is what we need to reinforce things from 35hz on down.
I have no objection to the F18 solution, and agree that the price point is lower than JTR. But relatively speaking, the price of either isn't that bad. If I were looking at "audio first" subs from JL or REL, I wouldn't even hit the basic price of admission for the cost of an overkill JTR. So it's all how you look at it, I guess. We're spending enough on the rest of the media system and room in general that the $2000 difference between a pair of RS1s and a pair of F18s isn't going to make me loose too much sleep. Another $3k beyond that for a pair of Funks is where I really can't justify it for our listening habits. Maybe for the bigger 2-channel system upstairs, but it's on a wood truss floor and does pretty well without subs down to 23-25hz, and won't be the movie location once the basement room is finished.
I'm still hoping that one of you has heard both brands and can offer comments on the difference in "house sound" when it comes to music.