I have atcsm40 want possible alternative

Nice pair of Acoustic Zen Crescendo ll available here now for $8400 that are more efficient and may do very nicely.  Best of luck. 

Try something that is more sensitive and can therefore “wake up” at lower sound levels. It would be useful to at least listen to examples such as (in the price range) Devore O/93. But... such speakers often sound at their best with tube amps (less damped), and I suspect they wouldn’t hit it off with your amp. ‘Haven’t heard the combo myself, but people seem to like the combination of Hegel and the bigger Harbeths. 

went with spendor d9.2   works well     thanks for all the input

Has anyone with older ATCs gotten the tweeter upgraded and found that the new tweeters improve low volume performance?  I've got a fairly recent pair, around 2012, but they still have the old tweeter.  They sound great when you crank them up a little but at low volumes do sound fairly dull.

Try Legacy Audio Focus SE. They play great at lower volumes and are easy to drive. There is a pair on here for sale.