I had a set of the ATC Mk1 to try out in my studio for awhile. While I found they sounded pretty incredible, I did have some trouble in the lower treble area, where I found a dip around the 4-5k range which made my work difficult to judge as I work with metal, and that's a crucial range for distorted guitars. These were a demo deal (aka "used") so it could have been the ferrofluid tweeter needing to be replaced, the Mk1's could have been 15+ years old, I couldn't tell.
I've been looking at other monitors that might work for me, and I keep running across the Neumann KH310's, and am wondering if anyone's had the oppurtunity to compare these two, especially with hard rock/metal? If I went with the Neumann's it would save me quite a bit that I could put on some hardware I've been looking at, so that is quite attractive.