I just received the amp. Thanks, Steve!
Initial impression: very different from the last C421 I owned. Maybe I had OPA2227?
Soundstage is larger. In fact, I think it's also larger than O2.
Overall sound signature is very clean and smooth.
Low gain sounds horribly compressed to me. Like playing back 64kbps MP3 without the distortion.
From memory, I heard better bass with E12, and also warmer, more textured mid.
Let's see what Steve has to say about E12.