ATC ACTIVE VS PASSIVE

I turn them on just for music - I have not noticed a warm up period but it would take an hour or so to fully warm the massive heat sinks. They do run warm but nowhere near as hot as a Bryston 4B SST just idling (Class AB). Each amp drives only one transducer over a narrow range of frequencies and I believe the active version has a 16 ohm version of their mid range driver - so power is not excessive and heat is quite modest. There are no passve crossovers so all the power of the amp reaches the voice coil.

@sm2727   

I believe ATC actives are biased Class A to 2/3 power. So they are Class A for the most part.

They are dated - discrete components and easy to repair and only minor tweaks over 30 years. Part of the appeal is that ATC got it right from the beginning. 

Unusual that the driver integration did not work for you - maybe your used speakers needed servicing.

ATC active is most important for their three ways. In a two way you could go either way (passive or active) with great results provided the passive amp is beefy (at least 150 watts on the SCM 20).