The Dunlop is a good wah, so it's not that. My suspicion is that Metallica would get a lot of their distortion by overdriving some heavy marshal stacks, or something similar. The only other thing I can think of is that you should try fiddling with the eq on your amp. Also, try using distortion after the wah, then another lot of distortion at the amp itself. There is little substitute for an overdriven tube amp though, which is generally not practical unless you are playing gigs.