AMT Legend Amps O2 Pre-Amp Output

AMT O2 being used as the pre amp into a Vox AC30C2. This pedal has 3 outputs: 1 with simulated cabinet, 1 as preamp, 1 as stomp box. Signal path: Fender BlackTop Tele - AMT O2 - return of effects loop of Vox AC30C2 - Shure Beta 57A - M Audio Fast Track Ultra - Logic Pro. Output volume on AC30 is very low, about 5-10%. No effects; totally dry except for Sony Handycam at 10 % volume. I am experiencing a buzz which I can’t eliminate unless I complete the effects loop. Any suggestions, comments, constructive c