Very interesting to learn of the specifics in implementation and your incentive using the Velodyne in the first place, thanks. The "sense of foundation" and "instruments nailed to the floor" descriptions in particular reflect my own impressions and goal using subs.
Again, very important info and findings here. How often haven't I heard or read of the ridicule in audiophile circles aimed towards those using such large scale sub setups; saying it's way overkill, only there to lift off the roof over one's head, showoff, blur one's vision, for über bass heads, crazy cinephiles, etc. Not saying such "verbose frivolities" (sorry) couldn't be the outcome with massive sub systems, it's a possibility for sure, but it's really missing the point of their potential (well integrated with capable mains) to create an immersive, hugely scaled, inherently powerful and totally effortless presentation.
"Overkill" to some is just an approximation of sufficient headroom to others, and just because the subs capacity is there in abundance doesn't imply one needs to run it +10dB hot. And yet, I believe it's quite apparent how many gain themselves (as in: negative gain) out of bass issues, be that response irregularities throughout the listening room with boomy peaks or subs sounding strained and thereby more distorted (even if you wouldn't expect it); lowering the subs gain can be a symptom of issues more than striving for frequency response balance, and thus proper bass presence may actually be lacking. The cleaner and more well-integrated the bass, the more gain can be applied without the lower octaves becoming too noticeable to get that more natural fullness and foundation in music.
I would definitely encourage you to proceed with a large scale subs setup, whatever bass principle route you end up choosing.
Given your info here I'd focus on a dual sub setup placed in their respective corners symmetrically to the mains. FLH's are big, yes, but it's also about how they're shaped apart from their overall volume. Another question is availability. Horn subs (FLH's) aren't readily available, but the TH varieties (patented by Danley Sound Labs) on the other hand are offered from pro vendors in a range of options. For proper extension and physical shape the DTS-20 could be interesting. They take up little floor space, but are tall (~87"). A pair of those, on in each corner, would be a powerful and extremely well-sounding solution. The DTS-10's are even bigger and take up more space, but mostly along the sidewalls - if such could be accommodated. The Funk Audio 18.2's in pairs also sounds like a potent solution.