Simply play your test music through a player with a multiband equalizer and play with the sliders.
This is how I 'tought' myself when I was young (used a real equalizer then which was hugely popular in those days.
Nothing beats those kind of experineces.

use a 'flat' headphone for this... HD600 or DT250-250 if you want 'flat' as a reference and play with colorations.

You're right indeed... I did that exercice sometimes bit I forgot. Sorry   

The problems I see here are a few.
1: dips are a lot harder to hear than peaks but lots of headphones have peaks.
2: sometimes sharp peaks preceded by a dip are hard to 'detect' or 'put a finger on'
3: headphones usually have lots of peaks and dips often exceeding 3dB and thus quite audible. It is difficult to 'break' those apart and one peak may mask dips and vice versa
4: resonances may not be visible in FR plots but can affect the sound, CSD and/or impedance must be included.
5: sometimes wildly varying HF can be caused by the measurement technique while in reality it is smoother. This could lead to an assumption of grainy highs where it may not be the case.
6: a sharp drop-off above 13kHz isn't detected as lacking 'air' all the time. A peak that isn't too high may be masking it.
7: Some people find LF roll-off not objectionable and say bass is tight yet another one, who knows what should be there, may find it lacking bass.
8: a peak in the 100Hz area with a steep drop-off in the subbass may still be perceived as a bassy headphone while it has no extension at all.

In short.. It is pretty difficult (if not impossible) to characterise a headphone's sound by looking at graphs.
Certainly not when graphs are taken by different people.

I have given up characterising it and know what to look for by now.
I like MY headphones to measure as flat as possible from 20Hz to 20kHz within +/- 3dB and this MAY get you a tonally balanced headphone but NOT necesarily a headphone everyone likes nor will it always sound good with any recording but it will with good ones.
The least possible resonances and 'echos' from the rear of the driver seems essential and is hard to see in FR and interpret from CSD.

I like the idea of characterising headphones myself and is why I made the FR plot where you can find substantial peaks at those points providing it is just one or 2 peaks that F up the sound.

Then there is subjective impressions that differ. One mans bass shy may be another ones 'real bass'.  One mans 'good bass' will be another ones 'bloat', one mans 'good treble' may be another ones 'I hear no highs' or 'piercing s#it'.
This too (perception) may be the biggest problem of all.

So my motto has become (for myself only) have an educated guess from plots when available while keeping in mind WHO posts the plots.
Read reviews and look for common traits in it or find one from a person who's hearing you trust.
But the best one of all is to audition one AT HOME for a while and compare it (one gets used to a presentation really quick).
Sell it on and regard the money loast as 'educational fees' or return it if possible and you don't loose out too much.

Just my POV

So I've been tuning the frequency response of my modded LCD-2 slightly, and it seems like boosting 4-4.5KHz by a very small amount can help a lot with snap, speed, transparency, etc...

Given that the LCD-2 is a bit de-emphasized in this area, perhaps this is why it sounds so dark despite having a spike at 10KHz?

Anyway, will investigate more.

Edit: not quite so. Darkness is something else. But 4-4.5KHz is definitely the edge/snap. I think "sharpness" is higher up (5-6KHz?) since I'm not hearing sharpness, but definitely a lot more snap, more speedy after bumping 4-4.5KHz.

CEE TEE recently introduced me to Adele's Chasing Pavements track, and I got the iTunes version following my meeting with him. Dear God, this recording is sibilant to no end.

Being a glutton for punishment, I had to check this out on Spotify. Ouch. Even with my HD600, which are pretty forgiving in this regard, there is painfully pronounced high frequency lift. I'm scared to listen to it through HD800's.