Like Jamie said, you're turning off the preserve primary color, which is telling the RIP/printer to "make" greys and blacks using CMYK instead of just black. So the banding issue is still there, it's just being masked by the fact that you're using three other areas of the head that are firing fine. It does look like you have *one* nozzle out on the K head, which usually I wouldn't even think about worrying about.
It looks like you may have a hair too much feed going. In VW, go to the printer settings menu, and on the feed calibration setting, note the default setting. Change it to use custom settings, and reduce it to whatever the default setting was, minus 0.2%. Try printing again. Play with this setting a little to dial it in. Increasing the number will make the printer feed more before printing the next pass, so if you have dark bands, go higher. Decreasing it will make the printer feed less before the next pass. You can have negative numbers here.