ATI AT4003 / ATI AT543NC for ATC SCM / Dyndaudio

Nope, I have not heard them directly.  I'm a Class AB guy, running fuse-upgraded Parasound JC1s, so I have no reason to hunt out ATI type equipment.  I have looked at the ATI pictures, that show a good power supply architecture in the AT543NC and variations.

auxinput: have you heard the ati implementation?

Replaced pass alephs with the ati, love it

The AT543NC uses the latest N-Core Class D amplifier circuit.  If you have not heard Class D amps before, I would suggest seeing if you can listen to one if you are serious about it.  I have heard Class D amps and they can be so extremely clean sounding.  However, they lack the high frequency extension and "air" that gives music life, so to me the Class D sounds somewhat artificial.  That being said, many people are not sensitive to this and just absolutely love their Class D amps.  Technically, I think the AT543NC is a very good implementation of the n-core circuit as it uses fully linear power supply (many still use switching power supply on Class D amps).

Like shadorne said, the ATI4003 uses the more conventional Class AB circuit.  If you don't know what Class D sounds like, I might suggest you go with this model instead.

 200 Watts is good with ATC. I am not familiar with ATI. 

The 4003 is more of a traditional audiophile design. Class D amplifiers are not yet mainstream for audiophiles but they can be brilliant. You have to consider that Class D, even if done to perfection, will likely inject more noise back into the rest of your system through mains and ground loops etc.