I listened to a pair of Grado 325's unamped a while ago though I am buying a pair soon...I found the grados to have some sibilance that can get irritating but I did manage to tame it with the treble setting...a good DIY amp could solve that sibilance problem...sound wise the Grados were amazing considering the fact that I love Dance, Alternative, Trance/Techno mostly some of the heavy stuff (compared to classical this is heavy, but my likes dont go in the heavy, heavy stuff like Megadeath, Black Sabbath etc. *ugh*) The sound was impactful I was totally taken aback with the music...I felt a sudden rush when every song started so Grados a must try for the kind of music I listen to.
I also have a pair of Senn HD-600 at hand now but I havent given them mush of a critical listen and I have to retur them soon
For the music I listen to I didnt feel much of that "in your face" impact but the sound is nice overall very liquid, transparent and clean though I found them too laid back for this kind of music. And the senns will need an amp to run at their true potential as they have a much higher impedance than the grados...grados will run straight out of a portable.
The senns for classical music were in a class of their own...the Grados sound colored for classical music and I felt immediatly that the grados are NOT for classical...senns do best for easy going music like classical
Frankly both would sound MUCH better out of a decent amp
Best idea is to get both for best of both world...if money is a problem then get a pair that suits your needs
Both will keep you happy for a very long time if taken care of....