It depends on what you're looking for specifically in a sound signature, whether or not you a different sound signature from the Grados, or stick to something similar. I've had the HD580(exact same sound as the HD600) and sr80i side by side before, and the HE-400 would sound relatively closer to the sr80i's, though both the HD580 and HE-400 sound quite different from the Grados. The HD580/600 sounds more mellow, with less of a treble emphasis than the Grados. Can't directly compare the HE-400 with the sr80i, as I've sold my sr80i, but from memory, they have less of an overall low end than the HE-400. The HE-400 have much more subbass than either the HD580 and the sr80i, while still remaining quite linear. The HD580 have more upper midrange than the Hifiman's (which is what makes the HE-400s dark, might be different from what you're used to). The sparkly treble at the top is probably the only thing similar between the HE-400 and sr80i. To me, the HE-400 have better technicalities (separation, detail, clarity), but the HD580 beat them on soundstage, with a more cohesive sound.
Right now, I'm liking the Hifiman's more than the HD580, but that's probably the new toy syndrome setting in. Keep in mind that the HE-400 are easier to power than the HD600. I've ran them off the Fiio E10, E17, JDSLabs C421, and they sound quite fine. So the HE-400 might be a little more lighter on your wallet, though not on your head. Too heavy for portable use.
I would either keep the Grados for home/portable use and get the HD600 for a strictly desktop rig (best choice IMO), or upgrade the sr80i to the HE-400. I found that after I got my Hifimans, there was no reason to keep the sr80i since they do what the Grados do much better. But then you would only have one headphone left that's for home use only, which would suck. The most important question in the end would be the type music you would listen to, I guess.
This might be better discussed on a different thread though, since it's a little OT. You're better off asking some more experienced members on the main HE-400 thread, since I've only had the HE-400 for less than a week now.