Franco....your post reminded me of this blog over at lensrentals.com:

http://www.lensrentals.com/blog/2011...th-bad-filters

Also, as I am currently evaluating square filters myself, stacking them is probably a good test.

BTW...regarding my first post above, I thought you were getting "fogging" with a single filter. Makes sense that it is multiple filters stacked (of if you did get it from a single filter, I'd return that filter).