Thanks for the input. I think I will like the zero coloration of the NC400, this is what I am looking for.
No problem
I'm not saying that the NC500's are coloured and non-transparent, I'm saying that you can choose the sound profile you're looking for based on input buffer opamp choice. The closest thing to the NC400 input buffer would probably be to use the LM4562 opamp. So far everyone who I've heard of comparing the NC500's with the LM4562 opamp buffer, to the NC400's, prefers them to the NC400's. But my personal favourite is the Sonic Imagery 994's. Super transparent with a touch of class A warmth.
You obviously like warmth, or else you wouldn't like the sound of the Gustard x20U, use a tube preamp, and amps with tubes. The Gustard is specifically voiced for warmth. Many people find the NC400's to be cold and dry.