I wonder if the tube is out of spec. tho? Did you check and make sure if it follows the tube or stays with the socket? It sounds like you did..just confirming
I had this exact symptom with my RM50...I couldn't match any previously matched sets...and my board was toast when I looked inside.
Difficult to get an exact measurement on components in circuit but these are easy to get to and check if you are comfortable with dropping the chassis out...it's right between the tube fuses if you look for the pics of my dead board or new board....
...please read on for the warning about lethal voltage (sorry, I shouldn't have just assumed you knew!)