So with my limited knowledge.. The issue is not with any additional hardware i use. And since the H9 in some scenario (not the send/return usecase i am looking for but in the master insert) operates with no hum noise, i am left to believe it has to do with the cables and the send/return connectors of the mixer.
I find it hard to believe that they are faulty connectors since i have used all 4 return channels (the mixer gives the option for 2 normal channels and 2 aux return channels) and the hum issue is there. So the only thing left are the cables themselves. Again i am no sound engineer but it has to do with that since i have used the mixer send/returns via internal audio interface and a laptop running ableton with no hum noise so also the internal wiring seams to be working fine.
