I have passive ATC SCM40 v.2 speakers in a 22 by 24 foot room, which replaced similar ATC SCM35 speakers. I replaced the speakers after 18 years of ownership primarily because the production of the tweeters was moved in house, and the treble became significantly smoother and less hard with poor recordings. Otherwise, they sound and behave similarly.
I initially ran the SCM 35’s using an Audio Research SP6B preamp driving a Conrad-Johnson MV75-A1. The C-J amp was not able to control the woofers well enough for my liking, and I brought a heavily rebuilt GAS Ampzilla out of retirement, which did a significantly better job of driving the speakers dynamically.
After replacing my preamp with a Cary SLP05 with ultimate upgrade and my Ampzilla with a new production SST Son of Ampzilla II amp, I upgraded my speakers. This combination drives this speaker really well and is what I would recommend. I have also driven the system with Schiit Freya+ and original Saga preamps, and would think amps like the Schiit Tyr or PS Audio BHK series would be great.
have also heard the active ATC SCM40’s numerous times and to my ear they have a sense of tremendous dynamic control and really excellent imaging and sense of inner air and space, as Lone Mountain (the US distributor) suggests. I think my setup approaches the active ones, just a bit less effectively. I wholeheartedly recommend both setups, but definitely with solid state amplification and tube pre amplification.