In the room with me right now I have a pair of: active 50's; active 10's and active 20's. I also have the SCA2 and CA2. I am not related to Lazarus28 :-)
I do not consider myself an ATC nut, I own other equipment and speakers too :-) Including a Paradigm servo-15 - which I happen to be using with the 20's right now.
Firstly, the 20's use a Seas Excel TWEETER, which it shares with it's bigger brothers (50's 100's etc). The 20 does not use the midrange driver of the larger speakers but instead uses a 6" ATC mid-bass driver. Enough nit-picking.
The room I have is 20x14x8, with a couple of openings into other rooms. In this room the 10's are a little bass light, the 50's are just right and if I didn't have the 50's, I'd be quite happy with the 20's bass response. The subwoofer is an "experiment" in acoustics and I'm crossing over the 20's at 40Hz.
The treble on the 20's and 50's is first class, the 10's use an older vifa tweeter which I thought was pretty good, until I heard the Seas unit.
I'm not going to eulogize the 20's - speakers are a very personal choice - to my hearing they are very good indeed. If you've heard Harbeth's you'll have some idea what your getting in to (I recently had a pair of compact 7's). I've had ref3a DiCapo's and thought the 10's better - again a personal choice.
As Lazarus28, the ATC preamps sound the same. You should decide based on whether you are going to use a balanced source, and want balanced outputs (in spite of its XLR connectors the CA2 is single ended) and whether you want more inputs and a more sophisticated phono stage. Balanced only really makes sense if your running long cables. That said, a benefit of balanced cables is that good ones can be obtained cheaply from mogami/belden/canare at guitar/pro audio shops.
One thing to know, these speakers weigh over 60lbs - they somehow feel heavier and need a robust stands - about 18" tall with a tidy sized top plate.
One more thing, if your are a "serial changer" of equipment, then ATC are not a good brand to buy new, you'll loose a lot when you sell. Try to get a good deal from a dealer if buying new, or watch the used section here..