Congratulations on getting your amp sounding the way you want! As you noted, it requires a lot of time and patience to try lots of different resistors and capacitors to find the best match for your system and your priorities, but that really is the only way. And it’s much more rewarding than blindly following advice on the internet.
As you said, new components go through a burn-in period before reaching their final state, and that makes it difficult to evaluate new parts. I use a burn-in jig to speed up the process. If you are installing a new resistor or cap in a spot where it won’t have DC across it, the jig is very simple. If it’s a low-current application like a resistor in the range of 1K to 1M, just connect it across the output of a line-level source like a CD player or FM tuner and let it play 24/7. For a coupling capacitor that won’t have DC across it, connect it in series with a resistor load on the CD player output. For parts that will have more current such as small value resistors or larger caps, you can do the same thing but connect them to the speaker outputs of an amp or receiver. For parts that will have DC across them, I use an old tube linestage and connect the parts in the circuit somewhere similar to how they will be used after the burn-in is complete, then run the linestage with a CD player or tuner so it has actual signal. I continue the burn-in for 75 hours or more depending on how quickly the particular parts break in based on past experience.
This may sound complicated but it’s not, and it saves a lot of time. I find the burned-in parts still go through a settling period once they are installed wherever I intend to use them, but usually it’s enough to just run the component overnight.