In short, theoretically everything that touches the audio signal, power (from the electric company to the component) or sound waves coming from the speakers matters.
In reality, only some are audible. I have done several tweaks - in my mind if it is not immediately noticeable, it doesn't register. Break in period makes it almost impossible to notice. Especially if it is supposed to have no initial effect. At least when they talk of speaker or cartridge break in, the initial sound change should be substantial and only improve from there.
It seems to me that the ones affecting the source are the most impactful, and it is reduced down the line of the signal as it is further distorted. For example, vibration control on a turntable that is not a highly controlled table like an SME, such as my Rega P8, when put on Townshend pods (seismic platform actually which I recommend more because you don't have to worry about positioning them - they come attached and fixed and individually adjustable for tension) sounded like a completely new table. When the same treatment was added to my solid state amp, I couldn't hear the difference. I put lesser vibration controls (read cheaper silicone feet) on that and my phono stage because I know they theoretically improve the sound, I just can't identify it specifically. I think that is what MillerCarbon does. He knows in theory it improves the sound and does whatever he can in that regard. It's a free country. I like to justify any expenditure. I find it hard to beleive he can hear every single thing he's done and when removed he would notice it. Double blind test would be nice to hear.
I appreciate honesty and transparency when assessing the impact of the individual tweaks. I was going to put some bass traps in my den as recommended by a member and in general. When speaking with GIK Acoustics, they reviewed my pictures of my room and because of the unique layout and placement limitations (and those of my wife) they recommended doing nothing when I spoke with him. I wanted to buy a couple things and the guy said for that small amount of treatment, I would not be able to tell the difference. What refreshing honesty!
I am trying to assess HFTs (under a different discussion) and would appreciate feedback on them from anyone who has tried them. They sound good in theory. This discussion seems to get big airtime, so I am hoping to get a lot of perspectives, especially from people more like me that have very good systems worth in the low 5 figures that blend into a non-dedicated listening room, not the ones that are huge, room consuming of mid 5 figures and up that are typically dedicated and set up for listening primarily. They look like demo rooms of audio stores, which is fine if that is what you want, all power to you. It would be nice, I just don't have the space/room that would make a good home for it.