Would you care to elaborate on why you were using 6 stop neutral density filter? I think I have an idea but I do not want to presuppose that it is correct.
Well the strobe I have doesn't have HSS, so I was stuck with the 1/250 sync speed. I wanted to be able to darken the background while also keeping the shallow DoF so I knew I would need some type of ND.
The 3-stop ND wasn't enough, especially at f/2, but the 10 stop was waaayyy too much. The 6 stop gave the best exposure/DOF control without having to sacrifice my ISO or shutter speed too much.