Here's what I love, all you guys that have never played the older Marks are in for a treat now, and seeing a whole new generation of players get to experiment with the volume 1 vs lead gain, treble pulled vs pushed, etc., is just awesome.
The early Marks through the Mark IV and the Triaxis really were something special in that you could dial in your gain staging on the amp at various levels with all the knobs and the push/pull options, and hey you could always run a boost out in front too if you'd like. You basically have three gain stages to work with with Volume 1, Treble, and Lead Gain. The later amps, the Mark V through the VII all have preset values for Volume 1, and while the JP2C does give you the pull gain knob that shifts Volume 1 from a preset 7 to a preset 8 from the original amps, it's not the same as being able to dial it in how you see fit.
So kudos to Mesa for bringing back this older style of amp, and I truly hope you all enjoy working with something so interesting and different than their more modern offerings.
Here's my challenge to you - don't use this amp just to play old Metallica riffs. Use these gain stage options and the super powerful graphic EQ to create your own new soundscapes. There is not 1 true sound to this amp, it's not just the Metallica amp, it's what you make of it.