There was an experiment recently here on the forum with
Starise. He was sending out individual tracks to a nice analog mixer. And also summing internally in a DAW. He was setting up the same mix on both systems. Not at first but once we got the mix refined the mixes were identical. We were comparing the stereo mixes and to be honest there was such little difference it was not even worth talking about. I was hearing the two versions here and we did quite a few tests and there was NO big thing in terms of how better one was over the other. We were not using the Dangerous though but the mixer he was using was pretty decent.
Often these summing boxes will either have transformers in the output stage or a valve makeup gain stage.
(Which often includes transformers too) Any differences you hear are more than likely associated with these. e.g. the Neve summing box has got output transformers in it so yes it will sound a little different. When the summing is purely resistive followed by a quality active makeup gain stage that is transformerless the differences are actually even harder to spot.
(in a controlled A/B blind test many would not have a hope!) There are other ways to get transformer and valve effects without using a summing box. e.g. plugins- console emulation etc..
There are cheaper ways to getting quality D to A conversion as well.