I run Marshall and they rock. Try this for your metallica set up, gain at full, channel volume at full, bass at around 7 or 8, mids below 5 and treble around 5 or 6. Use the overdrive or lead channel and if it has 2 overdrive channels then use number 2 as this is where the casscading gain is usually found. If the channel has a scoop switch then use it.