Hi gammalord, what do you mean with "swapping out power tubes section"? Replace the power tubes? I tried several times running "post pre" and the power tubes seem very quiet and efficient...except for the v5 fail, which seemed to resolve with a 12ax7 swap.
Impedance is right.
I will see another tech then... Could it be that it's not something directly related to the tube itself, but instead to the socket or to the solder joints that connect the socket to the board?
thanks for the reply!