Okay, so here's my situation. I am using a Behringer 4x12 cabinet with a Fender Mustang V head. However, I have a Boss ME-80 pedal board that I plug into the FX Send of the head, rendering all but the master volume settings useless on the head. (As far as I think, that is.) I am trying to get a solid, Metallica sound from the board, however I cannot seem to do it to save my life. More specifically, when I palm mute, it sounds very weak, and it does not pack any punch at all. I know for a fact that it is not my technique, for if I plug my guitar (ESP LTD Snakebyte) directly into the head, my palm mutes sound great, however it is not the tone I am looking for.

My pedal board settings go as follows:

OD/DS - Metal Distortion
Drive: 99
Tone: 0
Level: 50

PREAMP - Clean
Gain: 91
Bass: 27
Middle: 0
Treble: 88
Level: 50

Pedal FX - Off (acts as a volume pedal)

Comp/FX - Off

Modulation - Off

Bass: 48
Middle: 10
Treble: 95
Level: 63

Delay - Off

Reverb - Off

The delay and reverb I may enable after I get their tone, however I really need to find the tone first. Any help would be appreciated! Thank you so much!!
whack the bass up to about 60 and mids about 30 (they aren't as scooped as most metal bands). knock the treble down to about 80.

Thats roughly the settings I use on my head for a good metallica tone. Though you're using different gear it should be a good starting point
Not sure if you're using a tube amp or if it really makes a difference, but I have a tube amp and for Puppets/Justice I use a EMG 81 in the bridge and set mids around 4, treble around 8-9, and bass at 6 or 7. I have my gain at about 4. For the Load/Reload/St. Anger albums I use an EMG 85 in the bridge and boost the mids and keep everything else the same. For Kill Em All, Ride The Lightning, Black, and Death Magnetic I like to use an EMG 81 in the bridge and set treble to about 9, bass at 5, and mid at 6. Hope this helps, but again this is with my amp but I'm sure you can play with those settings a little bit
I have a Peavey Vypyr 15 watt and am having a similar issue. I'm trying to find a good sound for Eruption by Van Halen. Any thoughts?
way to much gain for starters. as metnioned you also seem to be scooping the mids way to much. your "punch" lies in the mids.

for VH you need a mid gain setting with some delay. don't expect to nail it with that amp it just isn'tup to it.
OP, I'll echo what others have said, you need some mid to your tone, it'll help a lot, I had to go through the same trials, in the search for my tone. Also don't use the highest output channel you can. I'm not familiar with your amp, so I can't speak specifically about it. However, I do know with my triple rec, switching from the 3rd (highest output) channel to the 2nd, and playing with the EQ, I found an even better tone, than what I was using originally. As for your palm muting issues, try using a distortion pedal, those are great at promoting the palm mute. I personally use the MXR Fullbore Metal, works wonders. Put your mid at about 50% or 12 o'clock, and work from there.

asethwidick - you need a higher watt amp, plain and simple. Don't be deterred though, we all had to start with a 15w amp sometime. I started with a Crate 15w amp, so don't be deterred, maybe get a nice 100w Solid state amp, or a 50w tube amp. Then you'll start to get the sound you're after
Wilkommen in der wirklichkeit

Esp Ltd RZK 600
Mesa Boogie Triple Rectifier SOLO head
Bunch of distortions, and overdrives
And a couple WAHs