@ei001h congrats on those ATCs! I have the SCM40 v2 and drive them with a Simaudio Moon 761. Stunning synergy. Up there with classic synergy like Luxman/Harbeth. The 761 shares the same MDCA tech as the 861 amp, which is what you should audition if possible. The 761 to my ears, is lighting quick, effortless in delivery, and just wide open across the board. Natural harmonic portrayal is a major strength (colors abound)! Sound is overall very authentic and natural. The 761 really brings out a special midrange from those amazing ATC drivers. For the massive SCM150’s I don’t think you can do any better for the 20K price point than an 861.
For preamps you might want to consider Backert Labs Rhumba Extreme 1.4, or Rhythm. I have the Moon 791 which is astonishing for an all in one streamer/dac/preamp but it’s probably likely you can get even higher performance from all separates. As they say, the sky is the limit as long as money doesn’t run out. Definitely check out the new moon amps!