I have been an ATC customer longer than I have been a Naim customer and can absolutely recommend them, and there have been many innovations over the years… of note with the newish curved box domestic series, the tweeter is hugely improved, over the earlier none in-house tweeters, and the crossovers significantly improved, so the newer models don’t need as much power to run compared to the earlier models which were power hungry and where the association of ATC and high power amps came from… new models a lot better… a little Nait can drive the new 19s for example.
The other thing with ATCs is neutrality and consequential enhanced emotion you get from them when listening to music with emotion… the conveyed emotion can be so intense ATCs are the only speakers that have moved me to tears or taken me almost into a trance… (with Naim electronics) … but this reality and transparency means you notice imperfections more elsewhere if present… so quality electronics and careful room coupling required.
For example the bass from the 40s can be over powering in smaller to small mid sized rooms, and that is where the 19s would then come in. But get it wrong the other way and the result could sound a little cold and distant.
ATC, along with some others, appreciate the importance of the mid frequencies, that is where much of the communication in music and speech comes from. The 40s have a dedicated mid driver, and the 19s have their famous combined very low distortion mid dome bass driver… a phenomenal unit that is used in their professional and domestic products alike.
So you do need to audition carefully with ATC to ensure optimum room coupling to really experience the magic from ATC speakers.
A word of warning, once hooked on ATC it becomes quite hard / impossible to escape, you keep coming back… I tried to escape to PMC, but came back, the treble in the 25 series just sounded coarse by comparison after a period of time, and the lower mid upper bass from some Harbeth designs sounded flabby and ultimately irritating by comparison… but these other designs I am sure would have sounded superb if I was not used to ATCs