I commented on ATCs in another thread...
They're fantastic speakers so long as you give them what they like - A ton of clean power, a lot of breathing room, and turn them up a bit volume wise.
If any one of these conditions are missing, they'll range from mediocre and forgettable at best, to dull, closed in, lacking and boring at worst.
Their imaging, sounstage, tone, balance, and dynamics are world class when powered right. I wouldn't use anything less than a 4B SST with their entry level speakers. Seriously.