FWIW. I recently got some grados as I was rather curious about them and could not resist to try them. The ones I got were are at the cheaper end, SR80e, so not reference/monitor quality by any means.
I've found them really nice for general music listening, also they work well with my soft piano, Ivory 2 American D provided I pick up the bass a little bit in the EQ, I really like then with the pianoteq demo also, but for the native Casio Celviano sound I didn't like them at all, not one bit.
I suppose it will depend on the digital piano sample, but in my Case the Grado sounds very thin for the Casio sound. While I don't know about the CLP 585, but many of the Cabinet pianos don't have an EQ option. The Grado Prestige series has a rather unusual frequency response, which is rather treble heavy and thin in the bass, that's exactly what I hear through them and the data here confirms. http://www.headphone.com/pages/build-a-graph
When I hook up the piano through the audio interface and apply some EQ adjustments I can largely address that, but straight out from the piano it really alters the sound in a way that sounds very different to how the piano sounds through the cabinet speakers, very coloured I'd say, and this characteristic is common in the more costly prestige models too.
YMMV whether you like them for your piano or not. A Grado reference phones are very expensive but no doubt would be better I expect.
In the end I returned the Grado as much as I would have liked to keep them for music. I also found them rather hard to listen for longer periods, fatigue being a major issue after an hour or more.
Many online stores will let you try and return if not satisfied with your cans these days at no loss, which is what I did.
It was a very useful experiment though alongside my other phones to have tried them and learned quite a bit from that. I found the links below very useful. As it happens I do find the response curves actually a pretty good indicator how a phone is going to sound after digesting the useful background given here.
A quick sum up what the various test can tell youhttp://www.innerfidelity.com/content/headphone-measurements-explained-square-wave-response-page-2
Background to how the measurements are done to mimic the response of the earhttp://www.youtube.com/watch?v=H0ClNJeHrAwhttp://www.youtube.com/watch?v=1uBuxqDSJlA