I don't know as much about the sound of the preamp, but to my eye, the M2 and M4 are some of the most beautiful and well made Japanese components from that era.
I have an M2 in my collection which I have not used very often but puts up a surprisingly good fight vs my Levinson 23.5 through my Magneplanars.
Others might want to learn more about your source and cables before trying to diagnose "raspiness".
Good luck.