I have had a similar issue on my Crack. For me, it's the left channel, and it's an audible hum that occurs most of the time. It is not dependent on the volume; it is present at any level at the same volume. The way I am able to try to resolve it was to manually tilt the 6080 tube. At certain positions, the hum disappears and at others it is amplified. I suspect a bad tube, which is unfortunate, but I've also tried the chopstick method, using dead RCA plugs, and trimming/reflowing a good chunk of my circuit. I also had passed all resistance tests. I received and built my Crack mid-June, just in case that tells anything about the source of the tubes.