I think acoustic music is one of those music that shines with anything
It just depends on which presentation you'd prefer..
This is all IMO but:
Grados- more initimate presentation, not as relaxing as most other headphones though
Beyer DT880- ultimate chill (sorry for the lame wording
), but it's just that, you can kick back and relax but it's not really as involving as grados
DT990- a mix between the 2, a bit more involving than the DT880 while still having the chill factor..
I think the senns that i've tried (HD580 and 600) are somewhere between the DT880 and 990
Oh and with female vocals, i prefer DT880>990>grados, in any situation
So up to you, but i find DT990 is involving while keeping clarity and details, so i'm keeping them and selling the grados (sold HD580 and DT880 earlier)