The fiio E7 is a great little DAC/amp, I have the E7 and the E7K myself.
However, using the Schiit Asgard with my Sennheiser HD 598 was a small revelation. The difference was not really mid blowing, but there was a certain fullness and richness to the sound that was there, but not so pronounced out of my MacBook Pro 13 inch without amping. I never had any Grados so I don't know how they respond to amping, so can't help there. Some amps do expand the soundstage, change the sound to slightly warmer or colder. In theory an amp should just amplify and not change the sound at all.
Again, especially the MacPro (made for audio/visual professionals)has great great sound components build in, so an entry level amp/DAC like the fiio might not make a huge difference. Also, for some, the tiny increases in audio reproduction can't be heard, for some they make all the difference.
It also depends on your source music and the quality not only of the bitrate/encoding but also of the mixing of the song. There are a few factors that make a difference.
I propose as the next step to check with a few grado users if they experienced a huge change when amped and which amp complements the grado the most. Second step, just try it out, maybe at a meet up or at some physical resellers if you have any. Other than that you might want to take advantage of the time you have to return things back if you are not satisfied. Schiit for example offers a no questions asked return policy.
Hope that helps,
Edited by Koolpep - 12/1/13 at 12:04pm