A lot to parse here, sorry fi this has already been answered. I know MA1 introduced dual 750 support awhile back, but is it still rudimentary? I believe, at the time, it was basically same settings as sub1 plus small tweaks like -6dB volume (something like that).
As a owner of both 310's and 120's, it's frustrating that even if I were willing to buy a 2nd 750 there's no way to leverage the DSP without dedicating the 2nd sub to the stereo pair. Ideally I'd have 310 as front L/R and 120 as surround L/R, both DSP enabled, but MA1 won't give me that unless my 120's act as a 2.1 which is rather overkill for nearfield surrounds (.75m). Do I have that right?
On page 10 of the 750 manual, there's an explanation of how the sub will accept uncompressed AES and lays out the subframe A/B channels, but it's not clear if the sub will accept a 7.1 signal or if it's just informational and you have to stick with a dedicated 2ch signal. If it accepts 7.1, even if the DSP were resource constrained I'd expect I could extract 2ch of my choosing and route those via analog out while using the sub for LFE.
Normally I'd just try it out myself, but Neumann.Control is laughably bad. I understand it's free but they should realize that app will be seen by most as a "demo" or intro to their paid offering and should receive R&D thusly. So, I'm very hesitant to spend $ on their software especially once you add in all the weird behavior I see with my 750/80's with network mode enabled. (750 won't come out of sleep, 80s won't stay in sleep, etc)