This is my first foray into tube audio, so i'm not really sure what is normal or not with this. I have the Crack/Speedball combo. My build issues have been resolved, everything measures out and it sounds great. Listening to it tonight I accidentally bumped the 6080 and i heard the 'thump' in my headphones. Tapping on the tube with my finger, rolling it around in the socket, or tapping on the chassis plate gives that same thump sound in my headphones. I would liken it to tapping on a woofer cone with your finger. Is that normal? Thanks.