i run 6 newpac's on GS hub @650 Mhz (autotune, so some are bit lower) no problem at all. I think u are having a power problem, try a more heavier PS with at least 10A
you should run a R306 per port, dont stack em on a hub or you will get a bottleneck, but maybe when u use an GS hub you can stack
As far as i can see your MB should be ok, when running linux on it you should be even more ok
Limiting factors: as far as i know usb-port, but you got a lot of them, so that should be ok

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@vh regarding latest version and 13 may version when using asic boost and windows 10 gS-hub 6 newpac's

13  may version using targets way to low. When i am using this version with --gekko-noboost, the freq go to 680 Mhz and after that autotune, when using it without this option target freq go to 150 Mhz
latest version: doing better, but freq stay behind @ approx 450 Mhz and some even lower

Switched back to 13th may version without asic boost after 30 mins, results of using it with asic boost are lower than without.