The 'fatigue' comes from an emphasis in the 4-6kHz range.
IME felt, toilet paper or foam can lower that range a little but only at the expense of a lot more upper treble.
EQ will help or, when not possible, a passive filter.
Lean... yes but strangely enough (well not that strange) lowering the 4-6kHz range reduced brightness and makes the overall sound perceived as a bit fuller.
It should be clear that the 4-6kHz emphasis works well in studio environment as it emphasizes 'details' and the Hi-X60 is intended for studio usage.