In your testing, you said you plugged the headphones into the audio jack on the computer and the volume was high. You then connected an RCA cord into that SAME jack and the other end you plugged into the ST2220T audio in jack. You then plugged the headphones into the ST2220T audio out jack. During all of this, you also had the ST2220T USB cable connecting the monitor to the computer USB? And the headphone is connected directly without the use of any adapter?