Tube amp bias question or not?

My KT120s have been in use for at least 3000 hours and are still going strong (over 2 years).

I bought a new (factory upgraded) JD502P a couple of years ago and a Radio Shack bias meter just to keep the Jolida LEDs honest, and they have been. Swapped out the 6550s for a quad of KT120s and kept the bias at 500mv, and they've been steady for over 2 years with minor drift mostly due to when I test them (typical drifty house AC)...well within spec. Running a set of great sounding Psvane 12AX7 T-IIs with NOS Mullard CV4024s (12AT7WA). I could get by with just the Jolida LEDs for bias but then the hair on my knuckles would grow back from not using the meter to burn it off.

The KT120s are larger so they have more physical mass (this isn't important really, but I like it), and they were so glowingly reviewed I had to try them. I doubt they add much, if any, output power (any amp has other factors determining output limits), but they do seem to have a certain dynamic quality where all the fundamental aspects of the amp's sound are revealed without seeming to be trying too hard. A wider range of somewhat more accurate tone from tighter bass to clear highs. That make sense? I have stuck the stock 6550s back in (or a set of Sovtek KT88s) just to see if the 120s are still up to par, and the 120s always go back in after a couple of days. This amp, which I rave about ad nasuseum as being a bargain in the tube amp world, seems to sound great with any of these tubes and personal taste could dictate sticking with 6550s anyway, but tube rolling can be educational and fun.

Jolida gives you 50mv of headroom anyway, and a dozen or so mv drift can be fairly normal.

The "blue glow" in my veteran 120s is gone...is this finally a sign of impending tube death? They still sound fine...hmm...

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.