dbot wrote:... No Analog Inputs connected to the RME => -100 / -99 @ +4dBU REF — -111 / -112 @ +24dbU REF...
... I don't really know what connected to Ground means ...
These figures tell that your ADI-2 Pro is OK.
"connected to Ground" basicly means a PSU's mains connector has a 3-prong plug of any kind, opposed to only 2 prongs (metal parts).
If I follow the description of what you got:
this setup is only usable with fully balanced interconnects, too many ground points for a clean unbalanced signal chain.
• So, first, make sure that every interconnection is fully balanced with XLR connectors and really balanced cables (2 wires + shield inside each cable).
• Check if the speaker's amp's power cords have a three prong or Schuko connector. Depending on the region you live in, the power cord's ground connector might look different.
Unlikely: but if the power plugs have only two connectors (= ungrounded) this would be an important information.
If the cabling is OK:
The only way to find the cause for the ground loop related hum that, following your description, obviously exists:
disconnecting everything except the speaker's amp.
Then replug one thing at a time, until you get hum,
• starting with the Mackie Big Knob.
• RME ADI-2 Pro.
• Then the USB connection.
At this time only the powered speakers should have a ground connection.
There should be no ground loop and no hum.
• Now plug one device at a time, starting with the one that is meant to work closest to the ADI-2 Pro.
Once the hum reappears you located the cause for the problem.
• Recheck if these cables are really balanced, see above.
• Have a look if there is a ground lift switch on the device, if yes try to change its position.
• Try temporarly remove Mackie Big Knob and use ADI-2 Pro for level control.
I cannot give more recommendation for a solution at this point, first we have to find the exact reason for the hum.
I suspect Mackie Big Knob destroys the symetry of the balanced signal.
Doing a continous, passive, balanced (symetrical) level control would need an extremly precise 4-way potentiometer - unlikely to have at this price point.
If my suspicion turns out true, either skip the Big Knob (ADI-2 Pro can perfectly replace it) or test the "Mackie Big Knob Studio" that's active and might work better.