Sounds like fun at the Warp Drive Warehouse.
Beginning with the basics, a shelf filter will boost as low as the FR of the unit providing the shelf filter. If the unit rolls off at 10Hz, so will the shelf boost.
Reason #1 to use the Bassis; it's flat to 2Hz.
Step 1) Place the subs all equidistant to the LP. Cancellation may result down low if the subs are throwing long waves from different distances to the LP. To check this thought, set the measurement mic at a point that's equidistant from all 3 subs and check that measurement against what you have already done.
Step 2) Get a close mic FR of all 3 subs and average the traces.
Step 3) Get a FR at the LP with no EQ.
Step 4) Adjust phase/distance to get the best FR without EQ.
Step 5) Set the Qs and Fs (F3) from the close mic trace on the Bassis.
Step 6) Set the Boost on the Bassis from the close-mic vs LP traces.
Step 7) Adjust Qb to fine tune the FR at the LP.
A 'modded' Bassis is any Bassis that's set up to your specs vs the stock Bassis. Here's an example of one that has fixed Fs and Qs, 3-position Boost selection, 2-position Qb selection, polarity reverse, 12-position phase adjustment, every 15 degrees, affecting only one of 2 (balanced XLR) outputs to reconcile time between dual sub placements and selectable 'none', 10Hz or 18Hz 2nd order HP filters for vinyl and other scenarios. This is optimal for operating my new Blackbird systems:
I only have the graphic as Phil is working on it now. I'm positive that a stock mono Bassis will easily tame your mammoth system. As I said, it's flat to 2Hz, so whatever boost setting you select will affect FR to 2Hz, and it has infinitely selectable boost to 24dB (256 times!!!). They are stupid accurate and can be used in more ways than the controls suggest.
I'm not a big fan of the idea that sympathetic vibration of a floor sucks out everything below 20Hz. That's just not a likely scenario.
Although you haven't mentioned the specifics of the measurement hardware, I'm certain that you'll have to raise the sweep level to get above the noise floor of the mic/pre/interface to see any reliable data below 20Hz.
I can lend you a Bassis if you think that might be something you'd like to try. I'm sure I have an idle one around here somewhere.