Lee filter holder with either lee filter slides or singh ray filter slides, both are considered to be the best manufacturers of filters and carry minimal colour cast. The problem you have is that you are using alot of light stopping power, the B&W is an ok filter but it still has a colour cast, couple that with a cokin and you are in trouble.
If you want a powerful high quality ND then use Lee proglass ND slides or the new big stopper, or there are the Singh Ray Vari-ND range of screw in filters. Be warned there is a huge leap in price from what you use now, but if you shoot with a 5dmk2 and 24-70 i am sure you will know the benefits of investing in the best equipment.
Hope this helps, if you have any questions just fire them back at me, i have gathered quite a collection of filters over the last year.