SCM 40 is a speaker I am totally ambivalent to in the entry series. While it is one of the most sold speaker in the ATC range, the CLD bass driver in it is not in the same league as the tweeter, the mid dome and the SL driver in the 19/20.
It always feels slow to me with some overhang compared to the super tight bass of the 19/20 and even the 50 which again has a superb 9" SL bass driver.
If i upgrade from the 19, I would stretch and directly get the 50.