So I have ripped up my cable management today, took the Aquaero and PowerAdjust out of their 5.25 slots and started testing with different configurations.
As I suspected, the PowerAdjust does only run on the Aquabus High (or Aquabus 2 as it's marked on the Aquaero), and so does the MPS.
When I connected it to the Aquabus High, I did see the PowerAdjust appear in Aquasuite as "Fan 5" but no matter what setting I tried, I could only get the fans running at full speed or not at all, even when I added it to one of the curve controllers, it's just on or off.
I then connected the Poweradjust back to the USB port and could control the speed of the fans with the slider in the PowerAdjust part of Aquasuite (only visible in USB mode), but I could not add this group of fans to one of my curve controllers because the PowerAdjust is connected over USB and not Aquabus

So let alone the problem with connecting the MPS and PowerAdjust on the same Aquabus, I can't even get the poweradjust alone to function properly on the Aquabus.
I have been trying and testing all day and just can't get it to work. What am I doing wrong here?
Edit: More weird stuff happening.
I have connected 6 fans to the PowerAdjust and am now running it connected to the USB port and controlling the speed with the slider in Aquasuite.
When I put the slider to 35% the fans draw 0,86 Amps at 4,3 Volts. It only takes a few minutes for the Fan Amplifier to reach 80 Degrees Celcius and the fans go to 100% automatically then.
I thought these things were built to drive power hungry pumps? Why does it struggle with 6 fans?
Even worse, the fans stay at 100% and do not slow down, even when the Fan Controller has cooled down to 50 Degrees.
Is my PowerAdjust defective?