See those little black spots in this crop? They're scattered all over your image. To get rid of these you just need more subs, four isn't really enough to get a good and clean rejection going.
I used to have a QHY9 which has the same KAF-8300 sensor as your SBIG. It is a great sensor but even at -25°C is it kinda noisy. Some more subs will smooth all of those little dots
With enough subs and dithering you don't even need to worry about darks with the KAF sensors.
Glen, I think it's caused by the Ha filter, I notice the same thing in my refractor when I am focusing on mag 3-4 stars with my bahtinov mask.