Weakened 300B tube signal

Dear audiophile friends: – the tubes (Golden Dragon) I am using in my VAC Renaissance amplifier are getting old (3000 hours). The sound level coming from the amplifier is lower comparing to the level when the tubes were new. What worries me is the change in the level of the sound: the high frequencies still sound “normal” while the lower frequencies are much reduced. Is this type of phenomenon normal? Your opinion will be appreciated…