I would add these two to "what matters":
- VST's. Many people are horrified at the idea of altering the "purity" of the signal, but you these allow you to tailor the sound to your taste. Examples, uBACCH, exciter VST's, MSED VST's, dynamic range expander VST's.
- eartips for IEM's. These make a massive difference. Although I can't measure it (I don't have the equipment), I can hear changes in frequency response, particularly in the bass if I get a good seal. A good eartip also provides more isolation from the environment and lowers the noise floor. In addition, some eartips are more comfortable than others.
- Many vinyl tweaks make a difference. There are too many to list, and I am hardly an authority on vinyl given that I don't own a turntable.
- Recording quality. This has already been mentioned and debated.
I would add to "what kind of matters":
- Computers. These need to be silent and preferably fanless so that they don't contribute to the audible noise floor.
And as for "what does not matter" - the list is virtually endless. Bybee purifiers, Shakti stones, Quantum dots, Peter Belt tweaks, vibration-free equipment stands and vibration "isolation", cable lifters, and so on. However, I would comment on these:
- Cables for IEM's. No, I am not arguing that they change sound quality. But I find IEM cables to be very important from a usability perspective. You want a cable that is non-microphonic, has memory wire for ears, has the right connector (I favour right angle connectors). With IEM's the most common point of failure is the solder joint between the IEM and the cable, so a detachable cable is an important feature for me.
- Audio cables are also important from a usability perspective. I have a habit of buying the cheapest cable possible, after all they are exactly the same the way they transmit signal. However, I have had a couple of cable failures, again at the solder point to the connector. Whilst this has (thankfully) never happened with a power cable, I have had two XLR interconnects which were not insulated properly short between the pins. This was devilishly difficult to diagnose because the problem was intermittent.
- "Audiophile" software. Any bit perfect music player sounds exactly the same as any other bit perfect music player. The only difference is usability and features. Some features make a difference, e.g. ability to host VST's and an ISO226 volume control, but otherwise they are the same.
I would suggest you separate out interventions that make a difference to sound quality, and interventions that improve usability. For many people here, Roon is not a waste of money, even though it is expensive and sounds exactly the same as any other bit perfect player, including free options like Foobar. But for them, that money is well spent because it improves usability.