Can you do anything to make power tubes last longer ?

I suspect you already had it. Don't leave on and less cycles. My  understanding is that tubes become less reliable... as in more likely to blow as well as have the sound quality slowly deteriorate. This is enough for me to change at recommended intervals. Three thousand hours takes a while even at the 2 - 3 hours a day, which I use mine. 

I wouldn't replace expensive and hard to get tubes unless they do something audibly/visibly wrong. But if you use new productions tubes, yeah, why not ?

This may not apply to your setup, but I have individual bias pots for each output tube. There’s a recommended bias range for each tube type . Running the bias toward the lower end of the bias range can supposedly get more life from them. It’s possible that the tube that runs at the lowest bias setting may last longer in general, but don’t know for sure. There’s no guarantee you’ll have optimal SQ with all bias ranges either, but that’s always subjective anyway.

Yes, bias is adjusted individually for each tube, and I follow the manual. There is no range stated, there is a black dot you aligned the needle with, that's all.

Be sure you know what you are doing in setting bias on the low side.  If the measurement is in mA, the lower number means running the tube more gently.  If the bias is in mV, a higher number means lower current and gentler treatment of the tubes.

Be sure the voltage at your outlet is not too high.  I’ve seen 125 volts and higher from some outlets.  A variac, or even better, a stepdown transformer, can lower the voltage to something like 115 volts or lower.  Running tubes at, or slightly lower than recommended greatly improves life of tubes.