Jeff Bagby did a writeup of the ATC midrange and included a lot of information about it. He thought it was about as good as it gets. I can understand why ATC wouldn't be everyone's cup of tea, though. Their overall balance is best at higher, more realistic volumes.
"As you can see the nonlinear distortion is extremely low, with 3rd order in the neighborhood of -70dB at my low crossover point. These levels are very close to the noise floor in my room and I would consider this to be in the state-of-the-art range for midrange distortion. "
http://studio-hifi.com/images/ATC75-150S_JeffBagby.pdf
"As you can see the nonlinear distortion is extremely low, with 3rd order in the neighborhood of -70dB at my low crossover point. These levels are very close to the noise floor in my room and I would consider this to be in the state-of-the-art range for midrange distortion. "
http://studio-hifi.com/images/ATC75-150S_JeffBagby.pdf