Modify tube amp or upgrade to NOS tubes

I am reasonably new to integrated tube amps, and have played around some with switching out the stock tubes for some other current production tubes - SED & JJ KT88s; Tung Sol 6sn7 & 6sl7, Grove Tube EL34 to get a sense of what differences in the music there might be. Putting all else aside - what makes more sense to those of you with more experience with tube amps if I want to go a next step - have amp upgrades e.g. resistors, capacitors, power supply; or get NOS tubes?

Thanks for any insights