I'm considering getting the B+W 3-stop and 6-stop NDs, and just stacking if I need more light loss. I get that you shouldn't stack filters if you can help it. But if it's the choice between two better quality filters (say a 3-stop and a 6-stop that could approximate a 9-stop ND filter) and three poorer quality filters (say a set of 3-, 6-, and 10-stop filters) that cost about the same, could the inherent quality of the better filters make stacking less horrible?
Also, how good are variable ND filters? "I heard on the Internet" that the interactions between the two polarizers can cause rough light gradients that should otherwise appear smooth. Plus, you've got 4 surfaces from the two polarizers anyway, so can IQ really be any better than stacking two high quality fixed ND filters?
Any other thoughts or experiences on how you configure your ND capability would be great. Thanks!