The Phonaks have tighter bass & sweeter treble, but the Grado's have better mid-range (guitars & vocals sound just wonderful) and a nice open airy sound. Sometimes the Phonaks sound a tad boring after listening to the Grado's. But the overall sound of the Phonaks is very refined and balanced, they can't really do anything wrong. The Grado's can sound amazing on some tracks (like Radiohead's 'Let Down') but fall apart on some tracks which the Phonaks deliver with ease (like Miles Davis 'Bitches Brew)
How are your CX300s working out? I once had a pair of Creative EP630s which i hear are quite similar. I liked them.