I suspect that you're boosting the wrong region. Most music (especially acoustic) has relatively little going on above 10 KHz and other than some very specific instruments like cymbals, frequency response at or above that range doesn't seem to make a huge difference to my ear.
Rather, I find that the presence region between 1 to 5 KHz can make a big difference in terms of perceived brightness, clarity and detail. If you look at Massdrop's
published measurements comparing the HD58X and HD650, you'll see that HD650 has more energy in exactly that range. Furthermore, the HD58X has more energy in the bass region, which can make high mids and treble seem relatively more recessed.
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So, if you want to bring the HD58X closer to the HD650, don't mess with the high treble and instead drop the bass a little and bump the presence region.
@solderdude also describes a
physical mod that you can do to lower the bass, which does in fact make the HD58X sound a bit brighter and more detailed. I personally actually prefer more bass, but that's because I listen at low volumes. Oh yeah, that's another thing ... in stock form, I find the HD58X's tonal balance best at lower listening levels. Push it too high and the veil to which you refer seems more apparent