I get fatigue when either listening too loud or if there’s too much treble. Try lowering the volume just a tad when feeling fatigue and see if that helps.
+1 to this, getting a dedicated headphone amp can improve the sound because of the reasons described, and perhaps also alleviate that fatigue if it comes because of an unbalanced sound (that may in turn be caused by impedance mismatch etc).
That said, I’m not sure I understand your listening setup correctly, you wrote earlier that you listen to the Marantz through an HRT MusicStreamer that then is connected to the HD58X?
What I can find on the MusicStreamer is that it is a DAC only, without an amplifier in it. Listening directly to a DAC, although possible, is not recommended (DACs carry a very high output impedance and are not built to drive current through a low impedance load like headphones).
The HD6XX is a great headphone, and as you like the HD58X I’ll bet you’ll like it as well. Especially since you’d like a larger soundstage, which, to my ears the HD6XX has. But unlike the HD58X, which will run fine on the headphone out of a computer, the HD6XX really needs a dedicated headphone amp to open up. An amp like Magni works fine, but it will improve with something like an Asgard or Lyr.
Aside: Yes, I’m talking a lot of Schiit here (Magni, Asgard, Lyr), but as you can see in my signature, I’m full of it.
Schiit I mean