Can you do anything to make power tubes last longer ?

Of course, not, just replace tubes as needed.

safebelayer, I think you can only use el34/6ca7 power tubes.

The system sounds underpowered in triode mode with my speakers, and I hear nothing positive, anyway.

Yes, it's 60 watts in Ultralinear and I think 37 watts in triode.

You just align the needle with a black dot, no red/green lights.

These Mullards are not excessively warm, though of course on the warm side. Just as my entire system is, including the speakers. So the match appears to be perfect.