Buy LEE or schnider optic.
Lee had some of the most variability in quality control and colour casts of all the filters I've tried their ND grads often have a purple cast in the darker densities. The B+W cast is very strong and like a copper tone. Very odd and quite harsh.
Heliopan are my preferred choice and offer more options in their range. I personally find their 5 stop to be the most useful. The Hoya 9 stop has a nice pink hue, which I really like too.