So what I know so far checking the internet :
- The PS500 has less soundstage, is more intimate sounding than the MS2.
- The PS500 has more bass, less treble. The PS500 is more warm sounding in general.
- The PS500 doesn't sound as good with guitars but is better with drums.
- The PS500 seems better with vocals.
Bear in mind that those opinions come from different persons with different tastes and not from me.
I wanted to know wich one was more neutral, but wasn't able find anything about it yet. Judging by these comments, I would say that the MS2 is more neutral.
Personnaly, I don't know wich one I would like better. Usually, I don't like headphones with big soundstage (hence my preference for Grados) because when I put headphones on my head, it's to hear my music more intimately.
I don't find the treble on the Grados fatiguing but I'm pretty sensitive to bass frequencies and I remember that when I had the Hifiman HE-400, I couldn't wear them for long because they had far too much bass for me. I just want the bass to be tight and fast, not deep.
Edit : grammar (my english is terrible).
The MS2i is a very neutral sounding headphone from my experience with once owning a pair.
By the way, they are excellent headphones. Iv'e never heard the PS 500, so I am not comparing the two, I'm just saying the MS2i is neutral.