How do I fix the muddy low end on my amp?

hey...any idea how to help fix my muddy low end on my amp?

I just bought a brand new Line6 Spider Valve MKII 212
(http://www.keymusic.com/gfx_productcode/109282/Line-6-Spider-Valve-MKII-212.jpg),
and it sounds pretty amazing at any volume to be honest, and the highs
and mids sound great, but I play metal, and I typically play a 7
string (Hellraiser C-7). I'm currently ignoring the 7th string
however, leaving it in G and the rest of my guitar is currently in
Drop C. Anytime I palm mute anything on the 6th or 7th string it
sounds pretty damn muddy and I can't figure out what to do to fix
it...The 7th string I could see getting a little muddy tuned that low,
although my previous amp handled it just fine, and it was nowhere near
as nice as this one. I've tried using all of the presets on the amp
and I've tried setting my knobs with maxed or close to maxed bass and
treble with no mid, and with no or little bass and treble and max
mid...everything still sounds muddy when I play those heavier
notes...please, anything I can do to fix this? I love the amp for more
melodic things and soloing and stuff, but sometimes I like to play
stuff a lil heavy too, and it's difficult to do so with a noticeable
amount of mud...
The low ends sounded great on my previous amp, a Line 6 Spider 3
75W...The Hellraiser has EMG 707's in it, and they delivered great
sound on my other amp...and I just recently replaced the battery for
it. Also, I've tried another guitar with Seymour Duncan
pick-ups...still muddy