I had the same problem, where my 4 QRXs introduced a low-level high pitched whine into my Sonosax mixer, not unlike what an IFB100 used to produce when on the same power supply. I believe at one point I even had the grounds (pin1) disconnected entirely, and this removed all of the noise, however this may have been specific for my cart/setup, and maybe not a very safe solution, for the equipment.
I recently switched to an RX12, and have since had zero problems (pin1 connected).