My Senn HD580's sound pretty nice on the 990. As justhavingfun said, the 600's do sound better with a good headphone amp.

The Grados are also nice, but leaky to the surrounding environment.

I find the Senn 580's/600's to be among the most comfortable headphones I've ever had the pleasure of wearing. They're circumaural, and sit on your head, not on your ears. I love Grados for sound, but they're among the least comfortable for me. Aftermarket ear pads can make a big difference.

You can find 580's for under $150 from legit sources if you shop around online.

Yet another forum: http://www.headfi.com/