Summary of the final conclusions.
On ADA1500 #3.
Have a second DSP sent to me to rule that out as a cause of the buzzing.
Also had XLR cables sent to me (the new, second DSP has XLR and RCA inputs.
After extensive internal ground cable swaps in the ADA and other suggestions by Engineer Andrew, likely the cause is a ground loop hum. It is oddly specific to the ADA and no other equipment, old or new, that i've had used over the years. In fact, of all the gear i've tested over the years, this is the first and only piece of equipment that is displaying a ground loop hum.
I can't say i'm 100% convinced, but using the XLR connectors instead of RCA definitely ended the buzzing.
I'll live with that most likely hoping the amp is ok and if i sell in the future, i'll sell it with the XLR connectors.
Just waiting for a final response from Axiom on it and the probably have the original DSP shipped back and then all should be done.
