I have 6600 and 6700 hubs which are basically the same and never had a problem. It almost sounds like one of the seal rings isn't flush against the bearings but I don't know how that would be possible unless one is damaged. I say that because I had a headset with the too loose/too tight issue and it was because the lower bearing race wasn't flush against the fork crown. Also happened on a BB30 crank because one of the bearings wasn't flush against the c clip inside the shell. I'd take the seal rings off and inspect.