I am the US importer of ATC, so know first that I am terribly biased. I won’t disagree with much said in this thread because opinions seem to ring true except @celtic66 - which makes me think he heard them in a terrible room. (They don’t sound like that to anyone I’ve ever demo’d to and I’ve demo’d to a huge cross section of people, skilled and unskilled, at listening). I do think they are clean, which is what low distortion sounds like, but the closer I get to people who actually write, mix or master recordings or are gear reviewers, typically the more they like the ATC sound (which is actually lack of sound of its own). Look up a review done by Neil Gader - passive vs active 50s- its interesting.
BUT the thing I can add to the comments above is 50As can do one thing differently than most speakers: reveal more details about what is front of them. I have never heard more difference in cables, DACs, footers, preamps, streamers, turntables etc as I have with ATC 50A-150A. Tube vs solid state preamps are MASSIVELY different. I think active 50s improve the hobby, make a better platform for experimenting as these little changes become even bigger and easier to identify. I don’t miss changing amps when changing anything else before the ATC 50A is such a significant change. I think 50 Actives are a blast to own over time for this very reason!
Brad