I have both.What does the Atrium bring to the table that the Caldera does not?
they have different strength and different chain for the best synergy to some degree. When i tried atrium on my friend setup which he tuned it in a way for biggest soundstage, atrium was magic, whilst the caldera becomes somewhat detached, less engaging. Conversely, my setup which is tuned for, say, 3d and more intimacy, the caldera takes the cake over atrium.
In my experience though which might be different than everyone else is that, caldera is overall more romantic than atrium both in my and my friend setup. (tube pre into SS amp).