I have had a DIY 300B integrated stereo amplifier since 2006 built by one guy.
But in all this long period of time I did dozents upgrade myself.
The amplifier has 6sn7 input, 6f6 (in triode mode) driver and 300B. Cathode bias and C coupling between stages.
I upgraded: output transformers, power supply choke, interstage capacitors, power supply capacitors, resistors, driver and input tube idle current...
My experience shows that big value power supply capacitors (after the choke), big value cathode capacitors, correct driver idle current gave me much better improvement versus change of output transformer from James Audio to Hashimoto H30 3.5 (3 times more expensive!).
Coupling capacitor quality and sound signature are very important too and cathode resistors as well. Also the quality of power supply capacitors is very important. To get big values and quality without spending too much I use a mix of electrolytic, polypropylene film and vintage German oil capacitors.
I don’t understand why producers put such small capacitors after a choke in 300B amplifiers (typically 100-200uF per channel). My experience shows x10 value gives huge improvement in bass control, transparency, focus, soundstage.
I don't have as much experience as @atmasphere @donsachs have. I also don't understand how audio designers make decisions. Probably a much cheaper change of schematics can lead to great sound improvement without spending too much on parts and size of amplifier.