Hi Matt, for a near first image that looks damn fine to me, just the processing that's letting you down.
IMO DeepSkyStacker is excellent for no hassles stacking but hopeless at final processing.
I've never been happy with the final image from DSS, poor colour & "blotchiness" usually being the problem.
After stacking, DSS creates an autosave.tif, this is a rather large 32bit/channel image.
If GIMP can handle this type of image I'd say use that for processing, if not then save the stacked but unprocessed image as a 16bit tif for processing in GIMP.
As Adam suggested, try just lights, lights & darks & so-on to find where the problem is.
DSS - Excellent stacker, terrible processor...
But that's just my opinion, seek professional advice.
Justin