well, idealy, a 40//15 ed ias jewel, but, it may b a better idea to keep it clean...
but, its ur choice...
personaly, i believe a nef rune is good for a bow, knockback is nice...
but, the extra dmg and ias may b what u need to make a really great ama
ur choice like i said, but i must say wone thing...

congratulations on ur find!