Could be imagination, could be a defective tube. Generally tubse last for 10+ years, BUT I have also seen defective tubes.
Take a known good 12AX7 and swap into the V1 position, test, then into the V2 position and test and then the V3 position and test. Did the results change with 1 tube position?
(Swap the tubes with the power turned off)
Engineer: Genz Benz