Hi there! I recommend you go with the 650's, they scale with good amp/source equipment very well and they're even upgradeable with cables/baffling and such. HD-600's are not bad too and they shouldn't be overlooked even though they're the cheaper ones. They have a different sound signature that isn't better nor worse and a large group of people here prefer the sound. I wouldn't say majority but there are enough of them haha. HD600's with Furutech cable and Furutech connectors are really good, the Cardas cable isn't too shabby either but you can get better made by a DIY'er or yourself if you're patient or ask for help around the forums.
I'm a Grado fan but they're very specific headphones for specific people, thats why they have like a 60/40 hit/miss rate and the people who enjoy them enjoy them like a LOT. I swear by them for my own purposes, but I seldom recommend them blindly to people. I mean I literally come home, pick up one of like 4 grados I have on me, and I spend 4-5 hours a day in them... but I wouldn't hand these out to friends since some of them want "bass" and "slaps" and crap like that. They're meant for natural music... I wanted to like these so much when I started listening to them that I would alter my taste in music to suit these but still not appreciate them. I gave up for about a year and one day I revisited them while listening to "organic" music that I actually was enjoying at that moment and it was love!
By the way, the Audio-Gd FUN is a splendid piece of gear, you'd have to spend like twice as much to ever get anything close to it. You should write about it!
