SCM20S is equivalent to SCM20SL, they were the first SCM20 to incorporate Biwire terminal and SL magnet. After that, all SCM20SL is basically similar. However, the S comes with beautiful Ebony veneer (which can no longer be had), and both side matched, a signature plague from Bill Woodman, and individually numbered.
The treble is not 'stand-out' in ATC because they are accurate. If you have bright front end, then ATC will be bright. Most 'audiophile' speaker inaccurately placed the soundstage too far. ATC places them just right, I can 'shift' the soundstage if I do something wrong in the upstream setup.
Used SCM20 for 4 years, upgraded to SCM50ASL. To add a bit of top end sparkle, get a pair of Totem Beak. 200W or more of solid state Class AB or 100W Class A mandatory for SCM20. Forget tube, even if you have high power like ARC Classic 120, it sounded mismatched, but Mcintosh 275 seemed to work fine.
Expensive, don't think so. I will prefer SCM20SL over Wilson Watt. Make sure SCM20SL sits on 21" high stand. 24" is too high, and lost the balance somewhat. Please throw away the blue pads if you are getting the Sound Anchor stand.
Anyone using SCM50, remove the rubber pads on the factory stand and replace with small globs of Blue Tack. The midrange will be much more transparent.