You know far more than me, but Ill reply anyway. First quote is mine and second yours.
Id say EQ can not change sound quality. An EQ only affects frequency response -- it can not affect anything else in the signal.
A little of this boils down to how one defines sound quality, which (at least) in this thread has no objective definition.
I have nothing to say about phase correction. Ive never managed to hear any in random music and in the tests Ive done I never manged to spot it. Ill assume youre correct though.
Frequency response is the most important part of sound quality. Think about it this way: you have a headphone that has very low distortion but a 10 dB bass peak. Perceived sound quality will be ruined by the peak. EQ the peak away and you'll get a much more higher quality sounding headphone.
I think its important to differ between subjective and "actual" sound quality, mostly because we havent defined actual sound quality. To me, sound quality is about accurate signal reproduction; its obvious that were not discussion that however.
Actually, I think it looks like we agree, apart from that I say "sound better" and you say "sound quality".
That's not a mind trick. Same as headphone with boosted or rolled-off bass are not mind tricks either. And you're not only changing amplitude, but also phase and therefore group delay just like the headphone driver does itself.
Here I disagree. Many people think cans with very boosted or very rolled-off bass are bad. Now you EQ to get a more neutral sound and all of a sudden they are good. Have we made the headphones better now? No, we just changed FQ. A mind trick. My point is that the actual quality of the sound hasnt changed, only your enjoyment of it.
I dont want to argue, but you mention phase alot. Could you show some examples of common music that has phase issues? Or cans that have phase issues. Im just a sucky noob with tin ears so I cant find any myself.
I've seen many graphic EQs with filters that not only work withing a single band but leak into the adjacent bands as well. Not what you'd expect by looking at the EQs user interface, but some EQs just work this way (without telling the user).
Hm, yes this is true and I should have included that in my post. I havent tried every software EQ on the planet but I think this is fairly common. You can check this with a spectrum and min/max a band to see how high/low its band goes. You might have a better idea?
*) kinda the wrong term since you can apply the gain after processing the signal with the EQ, provided you're using floating point (like foobar2000 does internally for example).
I dont really have anything to add to that, apart from that some software EQs (usually in media players) have a built in limiter to prevent clipping.
I definitely wouldn't recommend a compressor in a playback chain unless you know exactly what you're doing, and also not as a "better EQ".
Why not? With enough bands you can more or less use it as a flashy EQ. Its another tool to shape the sound to your liking. Its not like you can break something, only make it sound bad, and only temporary. You sure can make it sound really bad though (but you can do that with an EQ as well).
I think you should write an EQ tutorial because the most important aspect are the technical one/s. Shaping the sound is trial and error and learning what you like.