The Grado does not need an amp, but will benefit from a good paring with one.
The Grado is more detailed and open. It's highs are more piercing than the Senn. It is very light on the head and has a very natural/organic sound overall.
The 650 has a bit of mid-bass hump. It is smoother and has more of a "wall of sound"--although not to the extent of the SR Grado line or the Ultrasones. It's a "fun" headphone but presented in a way that does not contradict its audiophile quality.
I like them both very much.