I've had a pair of the 20s since 2015. I'm not a great fan. They are pretty good for rock, pop and stuff that needs slam and drive, but distinctly meuh with classical, especially baroque and choral.
I haven't found them particularly reliable, either. The amp packs obviously needed attention not long after I bought them. ATC supposedly fixed them (during Covid) and upgraded the drivers to latest spec, but they're still not quite right, and ATC don't really want to know. The speaker on the left channel seems to drop the tweeter after switch on. I need to up the volume to get it to kick in, after which it's mostly ok. Not disastrous, but annoying. It's been known to drop out during a listening session as well. They need to be played at quite a high volume to sound good anyway.
They're nice pieces of furniture, though, and the box count is low, which is why they're still here. Oddly, they sound best when listening from the next door room, the diningroom. I think a rega Brio-R driving the second iteration of the SCM9s overall sounded more fun.
For reference, the main system in the loft runs Quad 63s and Primare amps. The ATCs are run off a Primare Pre32 with the MM30 digital board.