Of course, much modern music is highly compressed and unfortunately you will hear that compression as harshness - more so with ATC's than most speakers, which tend to have a more recessed upper midrange due to the use of large 5 or 6 inch drivers in most designs.
If you want solid state then the Benchmark DAC1 or pre version is a good cheap option with ATC. I mention this because you'll need to ensure there is no digital grain in the midrange but without losing nice details - ATC's are similar to good horn speakers in sound and have some "bite" in upper mids. IMHO the Benchmark DAC1 (includes a pre in some verisons) sounds clean and smooth without losing detail and is good value.
If you are using analog or already have a nice clean source then either a tube preamp or some room treatments may indeed help.
Consider your room - 100's need some breathing space as they can easily get to extremely loud spl's - you really need to sit back about 8 to 10 feet to get them sounding best, IMHO. In the case of ATC's 3 inch dome you get very little "beaming" in the mids so the off axis response is nearly the same as the on axis whereas other speakers tend to be increasingly weaker off axis as you go higher in frequency in the upper mids. This is why I suggested room treatments - if you treat reflection points then this will reduce upper mid reflections and a rug or carpet will help too. Remember you hear roughly 50/50 or 60/40 direct/reflected in a room.