If your not using an amp for your Senn's, getting one will improve their punchyness and be able to have a more balanced Low, Mids and Hi's. If you want to clear them up and get rid of that Sennheiser "veil", you will need to change your cable. The 650's cable has somewhat been improved compared to the 600's, but will you benefit the most (for the darkness) from changing your cable. I have a Cardas for my 600's and it did for me. But I would definitely get an amp. Do you need it to be portable?
I would still look into Grado's. Their RS series can compete with your 650's, though they will have a different sound, and they too get the most brought out of them with Grado's amp. But if you look in the SR series, you'll get some of the best cans you can get for your money. And at 32 ohm's, you're getting great cans that can be driven buy just about any source (portable cd player, mp3 play, sound card...), which is not the case of the 650's.
I'm not vouching for any of these two brands in particular, as I own a pair of SR-225's and HD-600's. They both sound awesome, have their strenghts and sound.
I'd look into a good amp (can't recommend any non-portable one's as I'm not it that market yet) to help out your 650's and any amp that'll drive the 650's (300 ohm) properly, will do more than a good job on a pair of Grado's (32 ohm) if you decide to try them out.
Definitely get a high end replacement cable for your 650's before giving up on them (and give them time to burn in if you just got them, 100 hours avg.) and watch them amaze you when you combine that with a good amp and source. If that doesn't do it for you for the 650's, maybe the 650's aren't for you (i wouldn't rule out all of the other Senn models, as they don't all have the same sound, ex: HD-595 (50 ohm) don't sound the same as the HD-580's of HD-600 (300 ohm))
All of this, in my humble opinion
Spanky