@orange5o
Congrats. Yea, they may break in some, but, depending on what you want, changing the tubes will change the sound signature quite a bit.
Do some research, read the sound signatures of various tubes, then try some. It’s fun.
I also changed the 12au7’s to NOS Brimar’s from Upscale.
Pretty happy with those and the Dynaco/Telefunken smooth plates. I also listen to a lot of acoustical, jazz, and classical. So I wanted tweaks that would provide some ‘air’, as you say, sparkle, and definition. The stock Mullards are a bit thick and dense, and doubt break in will change that significantly. That’s what they are.
Then the SBoster LPS really made another nice improvement.