I have tried many different amps with ATC speakers and its quite easy to tell which are working and which are not. For example, I had a 130wpc Luxman 507u (I think), very expensive and feature rich amp. So much so I thought it could be my last amp, but it sounded absolutely terrible because I believe it hadn't got the current to drive the SCM 35's I had at the time and this was at any volume. The top end was very shrill.
I then put in the ATC integrated and everything came under control again. That said, I thought the ATC amp still had a sound to it which I didn't like and ultimately preferred a set of much older (the ATC was brand new) Michell Alecto mono blocks which gave a more natural if slightly softer sound.
In my view, there are sound differences whether the amp is working within its parameters or not. Of course, the age of the amp could have something to do with the sound too.