BTW, this thread is a perfect example of why I steer folks away from the forums. The odds are very high that you'll end up acting on really bad misinformation; typical of the internet in general. In this case from folks that haven't heard the speakers, have only heard an ancient model (ATC has been around since 1974), have heard a pair in less than optimum conditions or folks recommending amplifiers that they haven't tried with ATC speakers.
With that said, I have heard multiple ATC/amp combinations. Your best bet, as mentioned by several experienced posters above, would be the ATC P1 amplifier (retail $3749). The CDA2mk11/P1/SCM40 combo won best of show at the 2019 AZ Audio Video Club speaker fest against some stiff, in most cases much more expensive competition.
For greatest value and a very simple, clean system choose the ATC SIA2-100 integrated (retail $3999) with similar but lower power than the P1 and excellent built-in DAC. This is a very popular paring with the SCM11. Connect a quality streamer like the Roon Nucleus, subscribe to Qobuz and Bob's your uncle. I've heard this combo with both the SCM11 and SCM19 passives; outstanding in each case.
I also agree with a poster that the ultimate ATC experience is with their active models. But we just mention that as an aside since you already have purchased the excellent SCM11 speakers.
You have an outstanding, for life speaker. Continue to use your ears and don't let anyone else tell you otherwise.
