While its true that the greatest change in flattening nodes occurs when adding the first sub, adding subsequent subs also contributes to smoothing nodes, as well as increasing headroom and lowering distortion. I found that going to 3 from 2 subs further improved FR but required tweaking placement and phase again in all the subs.
The Harman study does provide idealized locations but the exact location in Randy's room will be influenced by the location of his furniture and its slightly irregular shape. That's why the SMS-1 mic technique makes finding the ideal placement a snap.
It also seems to me that if there ever was a case to be made for colocating a pair of subs, its Randy's huge room. One alternative is to colocate a pair of 350v3s for an extra 6db output from coupling. A pair setup that way would provide max SPL of 113 anechoic, 124 db in room
according to the Axiom stats. Along with the 600 that should give give you enough woofage for that 9000cf space and still give you some benefit of smoothing FR.