Steve is giving you some good advice here. I've listened to the SR-60, 80, and 125 and found very little improvement over the spectrum. The 60 is certainly a great value and the way to go in the Grado line.
I had owned the 590s up until about 3 weeks ago. I replaced them with the AKG K501's. The 501s are so much better, its not even funny. The 590s are not a bad headphone, but their highs are somewhat brittle, the upper mids a bit recessed, and the midbass bloated. The K501s are so natural, amazingly detailed, and musical, they're incredible. As a warning though, the K501s will have a drastically different sonic signature from the 600s that you've been using. I've found they work great for classical, jazz, and jazzier rock such as dire straits, steely dan, and even stuff like allman brothers. However, more straightforward and heavier rock tends to be a little bright and not quite dynamic enough.
I've heard the MDR-V6s quite a bit and yes, they are a nice headphone. And yes, they are accurate. Too accurate for my tastes. To me they seem overly analytical, not very musical. Treble is a bit harsh because of the analytical nature (not very smooth at all). For a "monitoring" can in this price range, I give the edge to the Sennheiser HD280 because of it's more laid back and musical presentation.
I have not heard, but from what I've read, the AKG K271S is similar to the K501, but is closed and is bettered suited for rock. This would be an excellent choice.
I've heard many praises for the Senn HD595 and 555. I find the bass too emphasized in the 555 so I'd tell you to go with the 595 if you went that route. Also check out the 580. It seems like different people have different preference when it comes to 595 vs 580.
So if you go Grado, I'd recommend SR-60. If you go Sennheiser, I'd recommend either 595 or 580. AKG K501 and K271S are both very nice cans, but I believe it depends on your listening tastes.
If you buy used or refurbished, you could get the Grados and a pair of Senns or the Grados and a pair of AKGs. This would be optimal cause you could use the Grados for rock and the others for other types. And you get the taste of which sound signature you prefer. Go to
www.headfi.com and check their classifieds section for good deals.