Just a 2 cents:
WE is a great name in tubes, this 403B had to be tested in a tube tester to say some clues.
If it is NIB, never used it may need run some hours to burn in and reach best sound.
If this tube had spend many years stored the cathode may take some hours of burn any impurities, and the getter clear some gas accumulated over the years.
I would emailing the amp manufacturer to help, as he know the amp circuit very well.
Good luck
