Forget distortion pedals, preamp pedals... if you want a decent heavy pre, get a Soldano SP77 cheap off ebay and use a decent tube power amp - Mesa and VHT make good ones.
The VTM120 is Peavey's version of a JCM800 - 1 channel, high gain only, limited tonally. I played one at a rehearsal studio once... its OK for thrash metal, not tight enough for shredding, I'd try it before you buy.
If you're buying an amp just for high gain, don't expect a great clean channel, unless you shell out for a VHT, Bogner, Diesel etc. Eventually you may want to buy another amp, preferably a blackface combo that says Fender on it, for shimmering clean tones.
About low end - for live playing, you usually want a rather chunky bottom for "chugga-chugga" type riffing - the Mesa Dual Rec w/ 4x12 does this very well, but the low end may become excessive when you mic it up in the studio - especially if you close mic it.
I have to say I'm partial to VHT amps for heavy shit (I've owned a Diesel Herbert, Dual Rec, Soldano). They have some of the tightest, most harmonically complex distortion tones I've heard. They also have a range of amps that go from AC/DC mid gain type sounds (50 ST), to classic metal (50 CL), to modern shred (Ultralead). And, my experience with the company's customer service has been truly enlightening (Your amps broken? Its 8 years old and you're the 4th owner? OK, we'll fix it for free type service).
They also have a nice tonal range thats uncompressed and very natural sounding - unlike a Dual Rec - you can get a great range of sounds by dialing in the gain and EQ... some also feature a switchable graphic EQ, which has incredible tone sculpting possibilities. If you dial in a scoop on the EQ the clean goes from a Marshall to a Fender type sound and the red channel takes on a whole new "evil" character...
Throw KT88s in there and watch out...
There are a couple 50CLs on
www.ebay.com right now...