I came across the same problem. It would not play with the 20db attenuator in place. But I never even tested it with battery mode. Only did a few test's with ac mode. I contacted John Kenny and he said try a different cable. I was using a radio shack BNC 75ohm. Then I got a Canare Double shielded 75 ohm BNC, but it still did not work with the 20db attenuator. He told me after the cable swap to try a 10 and 6 db together. But I have not tried them.
I have a couple of questions as well since you have a similar set up.
In battery mode did the 20 db attenuator sound better or different?
Do you like the Mk3 hiface does it sound better ? In what ways does it effect the sound? What usb and bnc cables are you using?
I was really exited when I got it and wrote a review to quick. (newbie excitement) My HPA was not fully burned in, which really effects the overall sound. In turn my test and review. Once everything settled in, I actually preferred just using the usb cable the hpa came with, without the mk3. It had a better placement of instruments and more relaxed detailed sound. With the MK3 in the signal chain the sound-field was better, but it became veiled and the bass was way to boomy and over emphasized. Using the denon 7000. It was a little better with the Canare bnc but I was disappointed. I did not want to cut my losses and I invested in a kimber select 2020 bnc and wywire usb. My first impression was what is wrong with you! Spending this much money in hopes to fix something, that by itself had a inferior sound. Now you have a REALLY EXPENSIVE dust collector. After very little burn in the sound has completely transformed. The bass is extremely defined and textured, in comparison to any other set up with the hpa. With or without the mk3. It has more detail and the best placement and that deeper wider sound field I liked. The cables have a long way to burn in. So I will wait this time to do a review. But I am now impressed with this setup.
Please let me know your experience.