I always test new tubes when I receive them, verify matched sets, .... see where the signal strength is for new compared to my existing used tubes ....
often, the newer parts are tighter tolerances than the older parts, i.e. +/- 1%; +/- 2%; +/- 5%, even back to the 1950’s, +/-10% was fairly common
audible difference? I replaced my capacitors in my AR-2ax crossovers as routine, without testing. I bought standard stuff from Parts Express, the originals were never exotic, and the 3 way design is what sounds so good, the crossover simply diviying up the frequencies as needed for the chosen drivers, renewed woofer surrounds.
Now that I remember, a new capacitor was a dud, found out when the speaker didn’t work. Parts Express replaced it without question, pronto. You would have known before starting the work. I have a multi-meter but never learned how to use it.
You have level controls, to inspect, clean/keep or replace, one pair I replaced, one pair I restored
I have some spare oem level controls a member here gave me, if you find the need
Wear gloves, don’t breathe removing the original fiberglas, new polyfill from Walmart.