I have always noted that when my output tubes were getting tired or sick I would start getting goofy bias readings.
Audio Research Replacement Tubes
I purchased an Audio Research Ref160S new from a dealer about 2 1/2 years ago and replaced my KT150 tubes yesterday at 3,100 hours (ARC recommends 3,000 hours) and am astonished at the improvement it made.
For those of you that may have some sound degradation or thinking about swapping out your tubes I highly recommend it.