Bad Tube? Which one?

Sorry to have to ask this question. I have been running a Rogue Cronus Magnum for two years. I've never tube rolled and still using all the original tubes. Tonight it's sounding muddy and distorted, even at lower levels. I stopped the CDP and checked the bias on the KT90's and they're all a perfect 35 mA. Does this mean that the KT90's are not the problem tubes and it's one of the 12ax7's or 12au7's?