Can you do anything to make power tubes last longer ?

In other words, I can set the bias as low as I want to until the point when I hear the deterioration of the sound quality. Setting bias by the ear not by manufacturer's recommendation.

The designer and builder of our monoblocks uses the same Dodd Balanced Power Supply to power his builds that we use to supply power to our system. They makes exactly 120 volts at 60 Hz. The 75 lb. laminated core transformer takes whatever comes from the wall outlet and makes a +60v and a -60v rail, both at 60Hz, and then joins them at the outputs. The amps are optimized for KT77 output tubes and can run no other type of output tube. I foolishly tried all of the usual suspects, even though I was told by the designer/builder that it wouldn’t work; he was right, can’t get any other output tube type to bias correctly. Had a lot of Octets for sale for a while :).

Anyhoo, I feel very lucky about the amps being designed and built based on the same power supply; apparently, pretty darn optimal. Accordingly, I’ll say again, for tube amps, it’s worth using a Balanced Power Supply (or, I think, a power regenerator) to assure that your amp(s) see 120v at 60Hz.

It's been a while since I ran amps that required biasing, but my approach was to start at the low end of the builder's recommended range.  Then, in part because I'm lazy and because I want to be gentle on the tube.  I would just leave things be and not bother to check the bias.  As the tube ages, the current flow drops on its own so the bias point drops.  By not changing the bias to compensate for the drop, i am actually preserving the tube, albeit with a small loss in output power.  

Put the Mullards XF2 in. Great tubes, probably need a little of burn in time. So much better than SEDs. Hold the bias perfectly. Should’ve bought them when I bought the amp. Noble, balanced sound.