That 2k-4k dip on your measurements on HD58x stock vs HD650- I would not have thought that degree of "subtly" would have made such a large difference in what I am hearing.
The 2k-4k dip is almost 4dB which is quite audible. Pull down the 3kHz slider on an EQ 4dB on a HD650, or reversely add 4dB in the HD58X just for fun.
On top of that the same is going on around 18kHz so you could play with a 16kHz or 20kHz slider as well.
I also use a passive filter that shaves of the 5kHz peak by just a few dB and is audible. That difference is much smaller than the 2-4kHz dip.
Sadly the 2-4kHz dip cannot be 'undone' using mods or passive filtering so selling or returning it seems the only way for you.
You are definitely not the only one not liking the HD58X though most complaints are about comfort.
Do you have measurements of your modified HD58x overlayed with the HD650? I also find it interesting that though the midbass on both measure the same it is the lack of the presence region on the HD58x that give the illusion of them having more midbass. I have always suspected when listening to speakers that are bright, it might (not always) just be they are lacking in bass and the treble response probably did measure decent.
When I do not forget the next time I amp playing with the measurement program I could overlay them.
I do have overlays with HD660S and HD600 (=HD580)
With speakers one also has to consider polar patterns and reflections/room thingies.
I do think Sennheiser/Massdrops marketing spiel that they are easier to drive is overblown.
Yes ! I read it in almost every review that it plays much louder... I measured the difference being just over 3dB which is not much but audible.
I do intend to try some of these mods as long as they are reversible.
Yes reversible. Getting the right materials and playing with materials to suit your taste is paramount.
The dip won't go away but may get relatively smaller.
I only like my HD58X modified and with the passive filter in line. I don't mind and even welcome a slight 'BBC dip' in the 2-4kHz range.