I'm not too knowledgable on initial sound acquisition, but I know how to tinker with it in post...
This question would probably be best answered in the audio forum.
Sound quality typically shows great improvement when going through a mixer, but you can also accomplish a pretty solid mix down in post, assuming you've acquired digitally, and you're using the right software, preferably something from Sony/Sonic Foundry.
Another major audio solution in post is Pro Tools.
