I'll chime in just because I own a few of the headphones discussed here:
7Hz Zero:2 -- Phenomenal for the money. I've bought several pairs, some for family and some for spares (which I haven't had to use so far!). The included cable isn't terrible, but I bought a Tripowin Zonie cable for about $20 because the original one started getting a bit stiff and wiry with use. Or maybe it's always stiff and wiry but it started getting stuck in weird configurations. I also prefer Comply foam ear tips for these, but that's more of a personal thing re: tastes and my ears. Either way, the Zero:2 is fantastic.
Beyerdynamic DT 770: I have the pre-X version (250 ohm because I use it with an amp), and I think it's a great headphone overall. I've never been bothered by the peak in the treble, but YMMV. Sound-wise, my biggest complaint would be that the low end can be a bit boomy. I don't really notice it with music, but I do notice it if I get a call on my computer and I'm expected to hear somebody speaking. The clarity gets a bit lost. Obviously that's not what this headphone is for, but it's maybe indicative of a loss of detail in the upper frequencies that's less obvious when listening to music. One thing I don't think I've seen mentioned yet about these, though, is that they seem pretty rugged. I would think they would travel well despite not being foldable. If you're not going to use an amp, there are lower-impedance models (and I think that's the only way the X version comes).
Drop Sennheiser HD 6XX: Terrific headphone if you want an open back. As others have mentioned, it's basically the HD 650. Just keep in mind that it's a 300 ohm headphone, so you might want an amp to get desired volume levels.
Any of those will sound pretty different from your Grados. I have the SR60 and the SR225e myself, and both have that typical Grado trait of peaks around 2 and 4.5 KHz. Some people like that very forward sound. Honestly, I've gotten to the point at my age where I have difficulty listening to them for very long anymore, so I don't tend to use them. I think any of the above will sound more natural once you take some time with them, especially the Zero:2 and the HD 6XX.