For me the 600 grilles just opened up the soundstage a tad. I was more concerned about the looks though as the 600s just look way better.
What's your current setup? I've always been somewhat of a bass-head and I have always wanted more bass from my 580s. I know most will take this as blasphemy, but it's good for a good 3/4 of the music I listen to...the other 1/4 is electronica which requires heavy punchiness that the 580s don't deliver sufficiently IMO. I am using a grubDAC prototype and modded Indeed hybrid tube amp.