I like Grado phones too but not much for rock; blues, vocal, jazz should be fine. Still they are a bit too soft for me and emphasize midrange. The higher you go in Grado line the better. 325i is where you want to start and move from there. As for headphone amp, it depends on how you want your sound to be and price.
I don't like Sennheiser at all though I haven't heard their newest phones. They sound "digital" to me as opposed to Grado.
I don't like Sennheiser at all though I haven't heard their newest phones. They sound "digital" to me as opposed to Grado.