I don't quite see the preamp-for-character in the mix bus thing, but if it works for you, cool. I'd say if you're getting a multi-use box for the job, get a mic preamp with line preamp functionality for a bit more versatility. But I doubt that you'll be using enough gain from the U5 to really bring out a lot of color, but I haven't used the box. Compressors, limiters and eqs tend to provide more options as "warm" boxes, IMO.
The RNC is wonderful, but I wouldn't consider it among these "warm" boxes, it just is a powerful, flexible compressor that won't wreck your sound.
da Bear