I have a pair of flat-pad SR-80s. I've only compared the Grados directly to the modded (that is, damped) YH-100s.
The first thing you notice is the big difference in efficiency, about 10dB, in favor of the Grados. Aside from that, the overall sound is surprisingly similar, but in my setup at least, I found the YH-100 to reach lower and play cleaner in the bass and stay smooth and transparent in the treble while the Grado clouded up a bit. These differences weren't huge, but they were obvious. You could listen farther into the music with the Yamahas. The Grados airbrushed the music. Both were pleasing. They cost about the same (I bought both used). I wouldn't want to give up either one.
I hope that answers some of your questions.
UPDATE, LATE 2007: By sheer cussed persistence and good Team Ortho work in coming up with simple, cheap reversible mods that tackled the problems of response extension, headstage and damping as they arose, it's my humble opinion that we've brought the sound of the best Orthodynamics and isodynamics to a point that far surpasses the sound of my Grado SR-80 and moves it up (with some reservations) into competition with the sound of all but the best electrostatics. Others may disagree, of course. Read on to find how we did it and what we learned along the way.
Edited by wualta - 1/28/11 at 4:00pm