Duane,
I have a couple of GH3’s and the Marantz PMD 661 that I use with a Rode NTG3 and the Audio-Technica AT4053b. I’m also often a one man band and I find this setup works well.
The Marantz has better pre-amps than other recorders in it’s price range and it’s simple and reliable to operate. The GH3’s have surprisingly good sound quality themselves so even though I almost always use the track from the recorder the dual audio setup usually gives me a viable backup track if needed.
I haven’t used the MixPreD but I think you could get great results with the Marantz PMD 661.
-Brian