I actually don't like the 600/650 very much (I like some brightness), so I'm not saying it's blasphemy to equalize it.
It's just that it's like living in a city full of burger joints, finding the one sushi place, and telling...
Any headphone or speaker that's very flat will sound "veiled" because all of the sound is crowded into the same levels. Just like erratic treble spikes give us a sense of clarity.
The entire point of the HD600 & HD650 is...
I found my DT 770s perfect for portable use when I had them.
That said, maybe check out the Sony V6? Cheap, easy to drive, bass is woofy without being absurd, treble is bright but not painful, very portable.
These headphones have been made in multiple countries for multiple decades and besides that suffer from the same quality control of a budget headphones produced by a global conglomerate.
I'm sure you would notice differences...