When I have run into the problem you're describing, frequently the source of the issue is that earth ground, the chassis, and audio ground are in a state of relative disagreement with each other. I just debugged an amp with this same problem the other day, and I had to scrape off some coating on a panel that was preventing the audio ground and chassis from connecting properly. In your preamp, I would look carefully at the tinned buss wire around the IEC power entry module and be sure all your hardware is tight. When you are connecting the Denafrips, it is temporarily restoring whatever is missing.
