Re: I want an 80's Metallica sound...
Which album? Kill Em All, Ride the Lightning, Master of Puppets, Garage Days, and And Justice For All all sound very different. Like some have said, the studio stuff will be very hard to emulate, but if your going to even get close to their recorded sound then what you use will be very different depending on what album you're talking about.
RTL was the last album that James said he used a pedal between him and the amp. I believe he used a Tube Screamer into a Marshall. After that, he says it was/is all strait guitar into amp for him. I think they all used passives until Garage Days Revisited. They for sure actives on And Justice for All, and I'm almost certain they didn't use actives on MoP.