Guitars and vocals span a similar frequency range so you can't really get a change in one without affecting the other. That said, I would give the Philips Fidelio S1 or S2 a try. They have a little more presence in the upper midrange where guitars have their "bite". They also tend to have a bit more bass than a regular GR07, which can be a pro or con, depending on the recording. Instrument separation is no better than with the GR07, though, and I really wouldn't call them an 'upgrade' from the GR07 at all, except that they seem like they might suit your needs better.