I think the consensus is that you can plug your external preamps into the preamps of the mr816, turn the gain down, and make excellent recordings. The colouration of the mr816 preamps is not detrimental to the source in any way at these gain levels. If you like to futz about you can run your preamps into the mr816 insert points and avoid going through the mr816 preamps, but then you have to run them unbalanced, and you will only be able to connect 2 at a time.
I run my 500-series preamps through the mr816 preamps with the gain turned all the way down (pad on), and I could not be happier with the sounds I'm getting.