Good question!
My uneducated thoughts. If there is something we don't like about the DP's preamps, it is too bad because we can't bypass them, other than recording elsewhere and importing a track.
I had a question about the quality of the preamps a while back, and there seems to be no complaints about them.
If there is something about an external preamp that you LIKE, use it! I frequently use tube preamps before the DP input in order to get some warmth, some vintage sound or some grit.