Tube amp bias question or not?


I purchased a Jolida 502P amp as an experiment back in January for use with my Magneplanars (1.7's) and it has been absolutely wonderful. Love the sound and much to my surprise my very powerful SS amp (CJ) is not seeing much use these days.

Now to my question. The amp has Tungsol 6550 power tubes and being that I am a tube newby I check the bias every couple of weeks or so. Three of the tubes have held rock steady bias - never varies - 500 mv all the time. One of the tubes tends to very slowly drift down to maybe 495 mv, then I tweek it up to 500mv and then maybe a month latter it's back down to 495 or so. What causes this? is it the tube? is it normal, is it anything to worry about?
128x128ejman
The "blue glow" in my veteran 120s is gone...is this finally a sign of impending tube death? They still sound fine...hmm...
Wolf I read this on the web it might give you some idea of what your are seeing:
http://www.rru.com/~meo/Guitar/Tubes/blue_glow.html
The blue glow is back (I have read about it someplace)...noticed it last night, so I assume it merely went on vacation for a while.