It wasnt all that difficult to get the Quantum 2626 connected to my combination of a Gigabyte GC-Titan Ridge, on my MSI Z370-A Pro motherboard and After some reading up, I have the 2626 connected and working.
However, when I connect my bass or guitar to an instrument input and there's a device (Tuner Pedal, Gate pedal, input/power module, drive/pre-amp pedal, tube input from my GS3000 desk, anything I tried, so far) in between the instrument and the 2626 instrument input, I'm getting a severe hum, about the same volume as the sound of the instrument.
When I fiddle with the connector in the 2626's input, the hum starts stuttering and when I find the right position/angle for the connector, the hum drops. It's still there, as a constant noise floor, that is well over the standard noise floor, I'm usually having from the devices I tried (a drive pedal, of pre-amp with high gain gives a certain noise floor, of course) and is still not something you'd start recording with. This noise also sounds different. Its not the higher frequency sizzle/crackle, like we know from a distortion pedal, but its a thick, low-mid frequency hum.
You might think:"This sounds like a bad connection, loose wire in the connector, or a worn out connector on the input" and yes, thats what it sounds like. But the 2626 is brand new and I tried multiple cables (high quality cables with Neutrik connectors), cleaned the connectors with alcohol, measured and even re-soldered the connectors from the cables, but nothing made any difference at all.
Then I used the same cables to connect my Glockenklang BassArt pre-amp to one of the2626's line inputs and there was no noise at all! Again with the same cables, I tried my guitar direct into the 2626's instrument input and also here, no noise at all!
This leaves me to conclude that the noise must be a result from having a device in the signal chain in between the instrument and the 2626's Instr/Mic inputs.

Anyone here who experienced a similar issue and can shed some light on the matter?