The amp connections are balanced XLRs so a flex wouldn’t be suitable but understand the point. I’d prefer to find a workaround than replace equipment. My set up does sound pretty good but I know it can be improved with some room correction and better sub integration. I could just use DIRAC but was hopeful to be able to use REW and fine tune further.
Hopefully there are some other workaround that others could suggest.
I don’t understand why the SHD can’t ‘see’/ isolate the 2x4HD/subs. There is a direct connection between them via RCA. Why is that any different to the connections going to the amps/mains via XLR. I was surprised this was the case and it seems a major flaw.
I did read that others have the SHD and 2x4HD connected via toslink so maybe that would be better?
The flex does have a balanced version. It uses 1/4" TRS connections, not XLR, but it's a standard cable for TRS to XLR male. But I understand not wanting to buy new gear.
So to be clear, you have PC -> [USB] -> SHD, then SHD analog 1&2 (or 3&4) -> 2x4 HD, and you also have SHD XLR 1,2 -> amp1 and SHD XLR 3,4 -> amp2, 2x4 HD RCA -> amp3?
I assume you have something like AMP1 driving the high/mids on speaker 1 and AMP2 driving the high/mids on speaker 2, then AMP3 is your subs. Are you bi-amping speakers or did you remove the build-in crossovers?
Because the SHD outputs are after the crossover and correction, you need to make sure the crossovers are passing whatever you need on the 2x4. The 2x4 would then need to apply its own crossover and undo whatever correction the SHD DIRAC did. You will also have timing problems, because there's no way to recover phase alignment. The 2x4HD is going to introduce its own delay after the SHD delay.
The 1&2&3&4 outputs on the SHD are AFTER the crossover and DIRAC, so you would be feeding the 2x4HD from the corrected SHD output, not the raw inputs. You either want to use PC usb -> digital coax (spdif) to feed the SHD and Y cable it to the 2x4 HD directly form the PC (e.g. Y cable coax, then coax -> toslink converter, or toslink from PC with a toslink splitter). That way you get raw signal to both units. Toslink might prevent some ground issues.
To summarize, I'd do it as USB -> toslink converter -> toslink splitter -> {SHD, 2x4HD}, and then manage them separately. I'd likely do the high/mids first (I assume on the SHD) and not correct for the 2x4HD frequencies, then do the 2x4HD and only correct up to your crossover frequencies. Remember you need to set all the crossovers first in the minidsp software, then run dirac. You might still have some timing problems with the subs as the 2x4HD and SHD will not run at the same speeds, but because everthing is parallel, you can likely correct that delay.
