im guessing youre going for a big shreddy kind of sound, so ill do what i can.
i dont know about the preamp question so ill leave that to someone else, but no, you dont NEED a big stereo preamp, im sure it would be a nicety. you could probably get away with the amps regular preamp too. most of the "metal" sound you speak of comes from EQing and a few select overdrive/distortion mixes. your POD probably has most of the stuff you need in it, but i dont know sh*t about PODs, having never used one. another load of youre sound would of course come from your guitar's pickups, im guessing a high output humbucker would be what you need. but since you asked for a high-tech, here i go
ill take a bit of a clue from KK downing of judas preist, since youve got a similar set up as it is
rack:
rocktron pirrhana
POD(i dont know sh*t about these, remember, but im guessing you can either mount it or run it through the loop) with an MIDI controller
Tuner
EQ(you could do a pedal too, but a rackmount is probably best)
head:
something HIGH GAIN, like a peavey, line 6 or marshall jcm or something with a good clean channel to run a dist box through(or you could use your POD for that)
Floor pedals:
wah, cuz wahs rock and you can fiddle to get em to add to your sound
Dist box, like maybe a line 6 uber metal or a DOD death metal, these are both nice dist boxes
a big multi effect, cuz multi effects rule. doubt me, i dare you.
and then the big thing for sound, EQ
for a super shreddy sound, you can scoop mids. this will always sound "metal" but it has too many downfalls at times
or you can boost the mids and highs, like i am currently. this will give you a GREAT sound if you do it right.
speakers:
412s are almost required. but, if you want to, im sure you could get away with a 410 and a 115. this is more youre choice than anything
i guess thats all i can think of off the top of my head
and since everyone else is doing their rig, heres mine
ibanez rg421 into a peavey bandit(or my marshall, but the peavey sounds way better) with an EQ of low-6 mid-7 high-8 on "modern" style. and sometimes i throw an external eq in and a flanger, but not always. yea, its that simple, and it sounds plenty