Yup I agree that the 6F8G is definitely more microphonic than my other tubes, however this is much different from the common tube quirks. This is like a really loud 150Hz buzz being played at 80% volume. The first time it happened I thought something had shorted within the amp and was genuinely concerned I had damaged my headphones (and ears) lol. Only happens with the RCA 6F8G with the volume knob is turned down fully, and as soon as I turn it up a little bit the noise is radio silent and music quietly starts to come through. Will try cleaning the pins again with some isopropyl alcohol, only gave them a quick brushdown when I first got the tube but that could definitely be the problem. FWIW, I did replace the volume pot with a stepped attenuator from eBay, but didn't think it'd be the problem since I don't experience the same thing with the other tubes. Edit: Very interesting development, cleaning the leads didn't help remove the problem, but now noticed that it only happens with my 1/4" to XLR adapter made from the original HD800 cable. I reterminated both my HD6XX and HD800 by adding XLR males and females to the cables. I get the buzzing sound to play through both headphones using the HD800 1/4" to XLR adapter but not with the one I made from the HD6XX cable. Will double check my soldering job, but really weird that this problem only happens with this tube and adapter.