If he bought it used (or even new), somebody may have put a funky tube in there to begin with or when it was sold (the unfortunate common practice of people switching out the nice tubes in their amps with junk before they sell them). My Genz Streamliner came new from the factory with one preamp tube that put out much lower than normal signal- I didn't even realize there was an issue with it until I started experimenting with tube swaps. It was one of a pair of identical tubes they use in that preamp and I could switch between the two tubes in the same position and lose nearly half the gain with the bad tube, but without significant added noise or microphonics- it was a pleasant surprise after thinking I had to crank the input Gain on the amp like crazy to get a decent volume. I also had a pre tube go bad after a couple years in my Mesa Walkabout that for whatever reason would only show its problems when fed a high impedance (passive) signal. If I put an active preamp in front of the amp, even with the volume cut significantly on the preamp it would sound great, but if I plugged in with just my passive bass the signal would be weak and would develop a louder and louder swishing sound while the tubes warmed up. A new tube in v1, and BAM- my trusty old Walkabout again, active or passive! I would have suggested preamp tubes first, but I assumed that would have been the first place someone would troubleshoot with preamp problems in a tube amp!
