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Hi! I just swapped out my nanodigi feeding a schiit modi uber to Pass ACA monoblocks with the flex. The bass and mids (driven by crown xls) are excellent, no noise even with gain on amps cranked up. The ACA and seos/b&c horns have a loud identical buzz, it sounds like a ground loop. Despite the high sensitivity they were quiet with the modi (which should be noisier than the flex). Noise occurs on all inputs and flex gain settings. 
-latest firmware
-grounded flex, tested ground
-swapped rca cables twice, rerouted 
-removed all potential noise sources from mains (system is on dedicated breaker box)
-tried switching tweeter outputs to outputs 7 and 8 (presently on 5 and 6)
-I would try swapping amps but my spare amps are at my parents house an hour away so hoping to resolve this without swapping.

Any thoughts on how to fix this would be greatly appreciated!

Swapped the tweeter amps 24v 4a power supply with an ungrounded 24v 5v. Quiet and smooth. It'll work until I get my audiophonics...and hopefully not pose a challenge for my fire suppression system heh. 
I hope those on the forum have a wonderful day!