You might have a power problem. I saw a situation once where a tube power amplifier drew enough power that when the preamp was on the phono, it was possible for the amp to drain the AC line so far that the preamp started to shut down. That caused it to put out a pulse which caused the amp to shut down a bit- then the line voltage recovered, the preamp made another pulse as a result and the cycle continued- woofers pumping. You could stop it by turning the volume down.
It would only do it with the tube amp that had greater draw and only when on the phono input (which has more gain and so is more sensitive to issues with voltage regulation).
If all the gear works in another room (and hooked up to each other the same way) then I would look into this. It might be as simple as a bad connection on the back of an AC outlet or in the breaker box. A digital voltage meter set on the AC voltage scale and plugged into an outlet on the same line might reveal this problem unless its associated with a particular outlet.
It would only do it with the tube amp that had greater draw and only when on the phono input (which has more gain and so is more sensitive to issues with voltage regulation).
If all the gear works in another room (and hooked up to each other the same way) then I would look into this. It might be as simple as a bad connection on the back of an AC outlet or in the breaker box. A digital voltage meter set on the AC voltage scale and plugged into an outlet on the same line might reveal this problem unless its associated with a particular outlet.