your existing or new AVR:
one way to reduce front power needs is to send the low bass to self-powered subs, i.e. keep power needs of front l/r lower, thus easier to use tubes.
IF using self powered sub/subs, fed from AVR, you need to know if in 2 channel mode, preamp front out, is the low bass going to the front or is the low cutout, still going to the sub/s? are they on in 2 channel mode?
new: go for an efficient speaker system so that the option of tubes is easier, and less power/heat is needed for those speakers.
IF efficient front l/r speakers, then you want center/surround of essentially the same sensitivity. then you can refine the output to each speaker individually, in the space
I would not let tube life be a concern, they last a long, long time, many 10,000 hours.
What I would do, to have confidence in existing and new tubes, is buy an inexpensive tube tester.
Since inheriting a big tube console in 1973, I check mine yearly, just prior to thanksgiving, and keep some spares, think long!!!