I have the same problem when using the E80CC tube with the TS 5998 tube. The loud hum / buzz is only there when volume is at 0%, goes away immediately when I turn the volume up. No such symptom when I run a regular 12AU7 tube with the TS 5998, or when I use the E80CC with the 6080, or when I use the stock tubes.
I have a 100k Valab step pot. Beyond the 0% volume, the music sounds wonderful without any hint of noise.
Lack of grid stopper? :^)
I had to look up Grid Stopper (thanks PJ my education continues) I often run the same configuration with the Valab fitted, I do get some noise as described above which is not present when running the 12au7.
however mine is more intermittent and seems be also dac and source dependent.
If I stream from pc to dac I have very slight noise ie the background is not black, when playing music this can not be heard.
If I listen to internet radio (dac feature) No noise same internet connection just different output source selected on dac.
If I connect pc via usb then no noise.
So could this be the frequency/oscillation aspect I see in mentioned in conjunction with the grid stopper?
What might the oscillation sound like? a very fast flickering?
I have been just changing my input to avoid this noise and was going to see if streaming via optical connection to my dac helped next. If the gird stopper might be a option it is something else to try.
Jamie