For me, equalization has made the biggest improvement.
Indeed. Now, if we can postulate that setting EQ to some value that immediately sounds "better" to you, what will happen to your SQ as you listen to that over time? Does it get better, worse, or stay the same? In other words, once you've artificially "fixed" the sound so your brain perceives it as improved, does your brain then
improve it more over time? Or does it leave a good thing alone?
In one of my setups (a laptop in bed), I have no amp or DAC, just my ATH M40fs headphones (which I consider the weakest in my collection). I have the music player set to heavily equalize the cans (something of a soft V-shape). It's good enough to watch movies late at night, so I can turn it up and not disturb my kids. Any music I listen to there, sounds funky (e.g., not as good), compared to either of my other setups (there are 3).
I brought one of my Bravo amps to this location, in the hopes that some weekend, I won't be working from home, and I'll have time to modify it. While it's there, I plugged it inline using a 3.5mm to RCA adapter (and still no DAC). Of course, the SQ changes with that amp in line. It makes the ATH M40fs sound significantly better, but overdrives the bass notes (because of the heavy EQ). So I need to re-equalize the sound (and make a new EQ setting, just for that amplifier), because it overdrives the system into distortion if I don't.
I am convinced that my brain would never adjust the sound quality to get rid of the distortion caused by adding the amplifier in line. In other words, my brain can't adjust for clipping. So I need to do that myself. After I get done messing with the EQ, the setup sounds more "solid" by virtue of the amp being there.
Bottom line: my brain can't equalize for distortion. It is unable to overcome a fundamentally flawed sound signature.