I'd like to add to this concept that EQ can increase perceived sound quality drastically.
I've always preferred a neutral warmish sound signature in headphones, and that's why I always gravitate to the Sennheiser HD600 series headphones. The only complaint I have with them is the lack of sub bass, but the tonality and evenness of frequency response is just unmatched. Even so, I use EQ with the HD600 mostly to bring down 3kHz and 7-8kHz, because those spots are a bit too forward. Using sine sweeps + EQ, there are some other frequencies that sound slightly louder than others like 2.5k and 11-12k, but if those are reduced with EQ, the sound looses some clarity. This is just speculation, but I think that has to do with my personal ear canal resonances.
I have been experimenting with EQing IEMs, and I have been trying to replicate the HD600's tonality in a portable IEM. I have done just that with the cheap Sony MH755, which actually has less distortion than the $150 Etymotic ER2SE. Almost every IEM I've tried has a hyped up response from 3-6kHz, and again, this is probably due to my personal ear shape, so I EQ that frequency range down a lot, around 6-7 dB in the case of the MH755 to sound like the HD600. My main method of EQing is sine sweeps using the SineGen program, plus looking at online graphs to set the bass to my preferred level. I try to make all frequencies a similar volume at normal listening levels, and reduce any large peaks. The Sony MH755 has a higher response in the 11-12kHz region that I only reduce by 2 dB because the HD600 has that same resonance, and that's why I think it sounds so natural and airy compared to most IEMs. That and it has a large dynamic driver with low harmonic distortion, many IEMs use BA drivers with loads of audible distortion.
So, now with an EQ setting personalized specifically for my ears using FR graphs and sine sweeps, I have an IEM that has extremely similar tonality to the HD600, with more sub bass extension. All in a $5 IEM. EQ is a game changer that everyone should use.
I should mention I also tried EQing the Focal Elex, and the results I got were disappointing. The bass and sub bass were incredible, but I could not fix the peaky treble even with EQ. I still heard some annoying metallic ringing which is that 20% that EQ can't fix.