I have been using 60 Hz "forever" but I am planning a little higher, probably 70 ~ 80 Hz. The Maggies go to 35 Hz but their distortion gets pretty bad when driven loudly at LF. As for me, decades ago when I participated in various studies I could localize to 60 Hz or so, but I have no idea if that changes over time or if in my current room (smaller than most I have had) it would be as noticeable. I have some room issues between 50 and 60 Hz (couple of closely spaced modes) so to take advantage of the extra subs I need to cover up to 60 Hz.
I would love to follow the guidance of Toole (et. al.) but unfortunately have physical constraints. There's a door on one side, and on the other I need the walkway, so there's not enough room to place the sub at the midpoints. I can pull the front subs out a little so they'll be about 1/4 of the way into the room. The rear subs are going to have to sit against the back wall. I'll let Dirac handle the little swarm as a (mono) group so all I really need to do is to get the delays reasonable for the front and rear pair. The subs in each pair will be symmetric, or pretty closely so, so I can treat them as pairs. Wavelengths are multiple feet so a few inches shouldn't matter. The front subs will be closer to the MLP than the rears so I will delay the fronts to match the rears. I may not need to do anything to the rear speakers (fingers crossed).
The XMC-1 provides 12 or 24 dB/octave slope so I will use its crossover (at 24 dB/octave). That allows me to get rid of my active crossover (and a few cables, which I just bought to go balanced all the way, natch). However, to get phase control, IIRC I'll have to enable the crossovers in the subs and set them to their highest frequency (hopefully out of the way).
I need to do a little more research, but AFAIK
I'll have to use just one of the sub outputs to get Dirac to properly EQ them. Knowledgeable folk in the XMC-1 and miniDSP threads say Dirac Live does not properly combine the response of multiple subs when it applies correction. Their advice is to run them as a mono set and adjust delay/phase per sub (or pair) myself. Many use a miniDSP but I plan to use the phase control on the subs. The XMC-1 has RCA and XLR outputs, and the manual says both are active at all times, so I should be good. I will use a Y off the RCA output to feed my current (front, PEQ2) subs. The XMC-1's XLR output will drive one rear speaker, with the second rear slaved off the first.
I don't think the XLR2 amp was an option when I got my first subs; Brian asked users several years ago and that was one of the features requested on the new amps. I think my current F12's are about six or seven years old; does not seem that long ago! Rythmik has been in business since 2002, incorporated in 2005, longer than a good many companies these days. I think I got my first pair when the company was still pretty young (and I like that it still has that "young" feel).
I could leave the setup as-is, with the fronts "large" and subs after my active crossover, but I am not sure room correction would work for cancelling modes unless I used "plus" (Emotiva calls it "Enhanced Bass"). I don't want to try that rabbit hole.
Should be quite the adventure... - Don