Bias Settings for Air Tight ATM-2 with TS 6550s

I have an Air Tight ATM-2 power amp which I bought used about six weeks ago on A'goN and want to roll some old stock Tung Sol 6550s (greyplates/no hole) into it.

About a month ago, I tried a quad of used 50s black plate 6550s when I first got the amp, but I didn't know then about slowly settling the tubes in--gradually increasing the bias over 4-5 hours. I lost a tube and a resistor in the amp, which I got replaced. I ran it with new re-issue TS 6550s for a month, to good effect, then tried some SED KT88s this past week, which were much warmer and mellower, though not as clear or focused or extended in the trebles. Now, I want to try some 60s TS 6550s which I bought used (about 100 hours) but they test very strong.

For about two hours now, I've been running the old stock greyplate TS 6550s at 1/5th the recommended bias (which is the low end of a "bubble" on the bias meter on the face of the Air Tight). At 1/5th the bias, four line increments below the bubble, the amp sounds terrific--big, mighty, mellow, fast, and with a warm string tone to orchestral music and good treble extension for choral music.

My question is this: why increase the bias to the lower end of the bubble (the recommended setting as I was told by The Analog Room) if the amp sounds good at the low setting about 1/5th of the way to the bubble? Am I risking any damage to the amp's transformers if I keep it at this low setting?

The only reason I consider this, by the way, is to shy away from the high bias settings which might stress these old tubes. The Air Tight is a high current amp and I've already had one old stock 6550 tube run away on me and blow. I don't want a recurrence.

Does anybody have any experience with this sort of thing? Please advise.