Good question...
The ASR answer to that would probably be: Those that have passed the highest level of the Harman training course.
I have not done so and therefor might not be considered critical enough or I did not find enough 'flaws' to be found critical enough.
I'm sure many would say these are too boring headphones
This is something I could agree with but the same can be said about the HD600.
The reason some may (but I don't) find it boring is lack of Harman type bass and no splashy treble.
The soundstage is narrow,
Yes, slightly better than HD6** series though. The driver is only angled 5.7 degrees.
sound is very in your face.
Yep I agree and think it should be for a monitor. Nearfield monitors (when used nearfied, as on a desk) are also very 'in your face' and as bodhi mentioned:
The sound is very neutral, it feels to me very alike the Genelec 8350
These Genelecs are 45Hz -3dB, the S5X (with good seal) are 15Hz -3dB though.
This becomes evident when playing music with subbass which remains very clean even at loud levels, only some Audeze orthos might be 'cleaner' at irresponsible levels.
Bass response is very good on the S5X which is evident on my (amateur) measurements and the lack of coloration there (no dip around 100Hz).
It could also be that the lack of distortion in the bass can show not so well recorded bass issues (at high levels).
There is bass, even slam but the lowest frequencies aren't there like with the LCD-X or even Sundara (both EQ'd).
The thing is.... the S5X does all this (for me) without any EQ in the bass.
Harman fans might prefer 1 or 2 dB more subbass and a bit less 150Hz presence.
Slam... a difficult thing. This is around the 100Hz band and too much of it or too wide bandwidth this becomes 'muddy'.
With speakers at higher SPL the tactile feel adds 'slam' and headphones do not have this tactile feel but needs to get it from slight midbass boost and high SPL.
The 'problem' here is that the elevated treble (about 4dB acc to me and my 'crude, flawed and incorrect' non industry standard measurements) makes you want to turn down the volume at high levels just missing out on the tactile feel.
Bodhi (looks like he does not consider himself nor me critical enough ?) also picked up on this.
With some songs I felt like there was too much detail, like the breathing of the vocalist was too clear
And this is quite true and might not even be picked out by a less critical listener. Certainly when he/she is used to say MDR7506, Beyer or AT or AKG studio headphones that have a sharp treble peak. This headphone has somewhat elevated treble but not 'peaky'.
For studio work this is an advantage as one can easily pick up on 'faults' but for hifi enjoyment it is a bit too much at higher (active) listening levels with some popular recordings.
Those listening to well recorded or classical will not mind that little emphasis at all (it bothered me so fixed it).
Also when listening at lower levels and with 'Harman type' EQ that small emphasis is welcome for most recordings.
I probably am a bit too critical when it comes to treble peaks. They are all too common in headphones but not 'seen' with industry standard fixtures.
I agree with bodhi that this is a good 'mid tier priced' headphone that has sound quality punching above what can be found in this and even higher price ranges.
Is this the perfect headphone ? No of course not. Too much clamping force, not the spatial feel from some other headphones, not as 'audiophile' qualities as some more expensive headphones.
But... those needing to use headphones to mix on or evaluate a mix or those that like the sound of nearfield monitors as they are (so no room curves applied) will find this a valuable tool that can simply be plugged in any device with a 3.5/6.3 TRS socket and does a great job without worrying about output power and output resistance.
For hifi enjoyment this is more than sufficient to my (not critical enough ?) ears.
Will it replace my (EQ'ed) HD800 ? No... (spatial qualities+ comfort) but should I not have any EQ available I would use this over say a HD650/HD600 or many studio headphones and even the Sundara.
I think bodhi is selling himself (and by extension me) short by wanting 'more critical' ears on it to verify if he perceived the headphone correctly and needs more 'industry standard' measurements on it with specific targets.
Those that know how recordings and live music (properly or not PA'ed) sounds like will very likely like this headphone as is, when they do not mind a high clamping force, which is quite common in studio gear.
This is and remains a
studio tool that (IMO) is also very suited for (critical or not) music enjoyment.