ATC SCM40As sound quality at low voume

The active 40’s sound better, or at least respond better at lower volumes compared to the passive 40’s I had but it might have been the gear I was using. It’s not so much that ATC’s sound poor at low volumes, they still sound good. It’s just that they sound VERY good at moderate to high volumes and it is easy to get carried away. I never felt they were lacking anything during late night listening. The amp being used was a Bryston 14B Cubed. Even my small bedroom speakers (SCM10 Signature Edition) were better at moderate volumes no matter what amp I used.

No issue at all. 

Anything as as accurate as ATC will sound great at normal listening levels or live concert  levels (ATC are impressive because they play cleanly at much higher levels than the majority of speakers).

However equal loudness contours mean that bass sounds perceptively too light at very low listening levels. This has everything to do with our ears and nothing to do with the speaker.

https://en.wikipedia.org/wiki/Equal-loudness_contour

That said, a speaker with excessive bass will ONLY sound good at lower than normal level. Again due to the way we hear.

shadorne is right - it's about accuracy.  It makes sense if you think about it.  Live music is pretty loud.  Music is written with live performances of real instruments in mind.  Making music that was originally loud sound good at low volumes requires the response to be modified to compensate.  This modification only works at low volumes, though.  Increase the volume to close to realistic levels and it sounds wrong again.

The solution, in my opinion, is to build a system that is as accurate as possible at realistic volume and then use equalization to modify it when you want something other than accuracy.  If you never want to listen at anything close to a realistic volume then speakers that are optimized for low volume listening are fine. 

I just bought a new amp about a year ago and noticed my speakers put a lot of bass out at low volume. It's around 300 or 350 watts into 4 ohms. I relaced an amp rated at 80 watts. Thought it was strange, used to have to put the volume up to begin to hear any bass. Now the balance of all frequencies stays present at much lower volume. I have no idea why this changed but would guess the increase in power. Other difference is my new amp is high current. It stays stable all the way down to a short. Easily drives 1 ohm impedance. There is no doubt powered speakers with active crossovers have much less distortion.

This is something i am very much interested in; that an amplifier that measures well into 1, 2 10 watts and above  measures(or sounds) as well at 1/10, 1/100,...etc.
And yet it's at these low outputs that many of our "audiophile" live and try to breathe.