I am afraid I do not have an answer. Grados give out colored sound. Audio-gd headphone amp does not color the sound, so they should be good match in theory.
You may buy a second hand audio-gd to try out before you commit any serious money.
I am a low tech DIYer, (tried to home made a Mapletree
Super II headphone amp and almost set fire on my apartment)
I admit I was impressed when I open up the Audio-gd headphone amp. They were so many electronic parts even for a $250 headphone amp.
However, they did not sound right when it get together with Grado. Of course, taste is subjective , your mileage may vary.
Another amp that gives me similar feeling is the expensive SPL phonitor, I would rather listen to my Grados straight out of Ipod.
You may consider to get small Cmoy pocket amp, it will gives you a lot of fun and it is cheap.
If you like the sound of Grado, try not to think of an expensive amp at the moment.
As I mentioned, Grado sound does not get a lot better even if you match them with an expensive amp. ( some of the amp do help a little bit on the bass and treble, but it is the strong Grado house sound takes main control)
I will spend the money on a better Grado instead. All the models from SR-80 to RS-1 has the same sound structure, some of them just have more controlled bass and better treble (or less harsh treble). Please note that the top models (PS-1000 and GS-1000) sound differently from the rest of Grado. You may need to try them out before you purchase the two top end models