I, personally, don't think there's another headphone that has a Grado-like signature. Grado, essentially, has their own signature, which many can't copy or have chosen not to copy.
Have you considered trying some other headphones outside of Grados? Or, do you consider your next step more logical staying in the Grado family and going with the PS-500 @ $599 or the next model up for $999? I think that's a hard decision that I can't make for you. Also, it's awfully hard to find shops that carry enough headphones and allow in-store demonstrations. Most, are still sealed in the original box and the store offers you a refund option, if you don't like them. But, even then, going through such an exercise with 5 or 6 different headphones can seem like a hassle at times.
Have you considered checking out an area headphone meet that might be near your area? If so, there's a good chance that you'd be able to demo quite a few varieties of headphones with different amps and sources to gain some more perspective into what you might desire.