In Australia, the hd598 seems to be similarly priced with (and sometimes more expensive than) the ad900, though I haven't tested them yet.
I've tried grados before, and I think they're too forward with the mids and highs for me. Not much other options shown on the buying guide though.
As for amping, I currently have an e11 which as I tested is adequate for powering the hd598, though I plan to get a desktop amp (and dac) soon.
There's nothing wrong with the AD900, but I'd go with the HD 598.