Decided a quick and dirty schematic wouldn't hurt:



A = Active
N = Neutral
E = Earth or Ground

The USB cable has 4 wires:
Data +
Data -
+5V
0V, which is connected to the motherboard ground plane which is connected to the PSU 0V which is connected to the EARTH via the case and the PSU.

I know the PAB and the Behringer mixer do not have an earth, I suspect the headphone amp (labelled HP) does not have an earth. These are OK as they are double insulated.

I know the PC does not have an earth when the 3 to 2 prong adaptor is used. This is the potential problem.

I don't know which of the other devices have earths, at least one of them must or there wouldn't be an earth loop problem in the first place. Those that do not are probably double insulated and therefore OK. (assuming they do not also have 3 to 2 prong adaptors.

If you follow the schematic you can easily see the ONLY earth connection for the PC is the USB cable if the 3 to 2 prong adaptor is used. This is unsafe. The USB cable will NOT have sufficient current carrying capacity in the event of a power to case type failure in the PC.

IMHO the best place to break the earth loop is in the USB connectivity, although the audio cable from the PAB to the Sub would also be a good spot PROVIDED NO OTHER OUTPUT DEVICES THAT HAVE GROUNDS GET CONNECTED.

--=-- My credo: If it's worth doing, it's worth overdoing - just ask my missus, she'll tell ya laugh --=--
You're only paranoid if you're wrong!