My experience with ATC SCM 12

You should hear the active speakers matched to their pre-amps. Much better representation of their true potential. They are not very forgiving of outside firm's gear, which tends to be emphasized for all the wrong reasons. Dull to warm. The active speakers have 250 watts of A/B power. I love their neutrality, and when compared to other speakers, they over-deliver on price to performance. I am upgrading to the Model 25s in 2012.

I wondered about ATC's philosophy re active Vs. passive loudspeakers:
What if you like push-pull tube amp amps, let alone SET amps? (A2 SET, not your flee powered ones).
Personally I find most solid state amps to sound a bit dry and not as holographic and/or emotionally involving as some tube designs, especially compared to SET's and OTL's.
I tried ATC 12 SCM with analogue source and all tube gear (45W/Channel 300B/805 SET) and got glorious mid range and very good response top to bottom with slight softening/roundness in the bottom end (not objectionable though).
In a smallish room, the ~60Hz lower response is deceiving as the bass is surprisingly deep and full and has no added artifacts due to the sealed box design.
I found that imaging and focus improved with deep toe-in (tweeters facing your ears). I would not call these speakers dull.
They just don't add "spectacular-ness" to the sound and are more "matter of fact" than most speakers which may be designed with "loudness" effect (emphasis on upper midrange and midbass to create a sensation of liveliness and open sound).
To my ears with proper source and amplification they are crystal clear without being analytical (dry) or peaky and they have close to perfect tone.
The 3" dome midrange driver is very, very good (full bodied, has smooth response and is very fast and detailed all at the same time).
So YMMV with room matching, placement and proper amplification but the potential is absolutely there.

I wondered about ATC's philosophy re active Vs. passive loudspeakers:
What if you like push-pull tube amps, let alone SET amps? (A2 SET, not your flee powered ones).
Personally I find most solid state amps to sound a bit dry and not as holographic and/or emotionally involving as some tube designs, especially compared to SET's and OTL's.
I tried ATC 12 SCM with analogue source and all tube gear (45W/Channel 300B/805 SET) and got glorious mid range and very good response top to bottom with slight softening/roundness in the bottom end (not objectionable though).
In a smallish room, the ~60Hz lower response is deceiving as the bass is surprisingly deep and full and has no added artifacts due to the sealed box design.
I found that imaging and focus improved with deep toe-in (tweeters facing your ears). I would not call these speakers dull.
They just don't add "spectacular-ness" to the sound and are more "matter of fact" than most speakers which may be designed with "loudness" effect (emphasis on upper midrange and midbass to create a sensation of liveliness and open sound).
To my ears with proper source and amplification they are crystal clear without being analytical (dry) or peaky and they have close to perfect tone.
The 3" dome midrange driver is very, very good (full bodied, has smooth response and is very fast and detailed all at the same time).
So YMMV with room matching, placement and proper amplification but the potential is absolutely there.

I use atc scm20, and like others said, the amp is just not up to par. Put them on some high bias class A or high current amps like byrston, and you will hear what the atcs are all about.

I've been using the 12's for about a month. Bought used so broken in. I concur that bass was lacking and they were flat... that is until I spent a week placing them. Now they are anything but that. Amazing monitors -- very 3D, neutral (in a good way, ie engaging with the full picture being presented in an unbiased fashion) but bass is the standout here. Their specs are conservative, I'm hearing as much quality bass as I had with previous speakers rated at ~40hz. The point is that proper set up is key, and room treatment. Doubt anyone would be impressed with them otherwise. Tip: Toe them out more than you may be used to and a bit closer to the wall. With those caveats, these speakers absolutely perform. Driving them with 250 watt @ 8 ohm Class D monoblocks.