Can you do anything to make power tubes last longer ?

"Next year maybe, I spent enough already. I would be cautious to take the sound in  warmer direction compared to how it is now” yeah, next year you will be one year older, and hopefully everything around you will be “warmer” LOL

@inna 

That was one of the reasons I went with Hybrid stuff. I currently have PS Audio.  The BHK preamp that uses two 12AU7’s and the mono blocks are currently using 6922’s.  All of which can be affordable and last a fairly long time.
My downstairs mono blocks use 4, 6HB5’s in each and they starting to get expensive to replace. I’m still using an ARC SP9 MKll Hybrid preamp with them and it uses 6922’s also.

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

Now I wonder how Mullard XF1 and the earliest ones with metal base would sound in my amp..