I just thought of something else. With 3 mm filament, variances in the diameter of the filament have a much larger effect on the actual amount of plastic extruded. We've seen the diameter in different places on the same spool vary but up to 0.10 mm. This means that you could be pushing out 10% more filament (by volume) in some spots than in other spots. I wonder how much of the Z banding we see is actually caused by differences in the filament diameter.
I bought a Rostock MAX v2 kit from Matterhackers and assembled it. I never built a 3D printer before and it took me about 30 hours. I had to solder the heat plate components (led, thermistor and a resistor) and a few wires. The assembly instructions are very good and easy to follow. There are checkpoint videos on YouTube that help you validating each milestone of the building process. I am very pleased with its performance and the quality of the components. SeeMeCNC's and Matterhacker's technical support is very responsive and knowledgeable. Most likely my second 3D printer will also be a Rostock MAX.