The Fidelio X1 is Philips‘s flagship headphone. The build quality is excellent and the looks are really great.
The picture just doesn’t do it justice.

The used materials are very nice and seem high quality materials.
The detachable cable (ordinary 3.5mm stereo jack) is a good thing. The headband is very soft and comfortable. TheX1 has velours pads.
It is clearly intended for home usage even though it has a relatively low impedance (around 30 Ohm). The give away is the fixed (nice looking) 6.3mm stereo jack and thick long cord. There is a 3.5mm converter but you really do not want to use such a heavy construction (big adapter, big 6.3mm plug and thick cord) plugged into your DAP.
It is an open headphone which means that
A: you can still hear what’s happening around you and
B: others around you can hear what you are playing very well.
So far only positive remarks. BUT there are also some negatives. The velours pad material isn’t as soft to the skin as other velours pads. Directly out of the box it smells considerably (a bit like those NASA foam pillows) but reckon this will go away after a few days. It won’t be with me long enough to know this for sure though. The reason being; my expectation which may have been a tad too high.
Another thing that annoys me is the cable which is a bit stiff and is microphonic. Fortunately the cable can be easily replaced.
The little hears on the velours pads aren’t as ‘comfy’ as Beyerdynamic ones.
Below the frequency plot of the ‘high definition’ Fidelio X1 (red = right, blue = left)

Everything from 30Hz to 1kHz is well balanced and sounds good. Also the treble part isn’t raised which ensures it doesn’t sound shrill, sibilant or harsh.
The problem area is between 1kHz and 6kHz though. There is a dip there which takes away a bit of the sharpness of voices and instruments which can be a pleasant experience with some (harsher sounding) recordings.
On well recorded music it takes away too much and makes the X1 ‘darker’ sounding but without bloat or heavy bass.
The treble being present again masks this a bit as you do hear the treble in the proper amount.
Another downside is the treble
extension. it drops of quickly above 17kHz (on the left) but on the right it drops off quickly above 13kHz.
In my opinion it doesn’t deserve the ‘high definition’ markings on the nice looking enclosure.
It isn’t high definition and aside from some ‘clarity’ that is missing it also looses out on the 'finesse' in strings and cymbals etc., it lacks ‘air’ and 'high definition'.
I would like to have said it is a reference grade headphone but it is NOT.
Even though the looks and quality appear top notch the sound is good to decent and a bit darkish (lack of clarity) but with the right amount of treble.
Definitely NOT flagship-type sound quality and would simply say to pirates ... skip this one

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The CSD below shows little to no ringing which actually is good thing… yet… it can’t convince me of it’s purported greatness.

Verdict: a nice looking and well made headphone that screams ‘high quality’ but has some let-downs on the audio side of things (missing clarity and top-extension). It is NOT a bad sounding headphone but also isn’t a very good sounding headphone.