Can you do anything to make power tubes last longer ?

Yes, not inexpensive. I made approximate calculations. Unless prices for NOS or near NOS tubes go much higher I will need about $40/$50 per month on tubes. This is no problem. But I need only four power tubes. Excellent NOS 12au7 tubes from a reputable source or ebay can be had for about $50 each. 12 AX7 are often two/three times more expensive. Well, the NOS 12AX7 long plate Mullards that I have in preamp were $450 for a matched pair. But they should last for a very long time, so no worries.

Coconut oil