Heres a fresh idea for the board...try one out before you dump on something online...maybe this will start a revolution...and maybe someday people will read these threads and say...those guys had integrity and they wasnt just full of hot air.
I just Googled "Line6 Mark 3" and nothing came up. Do you mean Spider III or Spider Valve, by chance? If so, I've played both - the Spiders are practice amps, nothing more.
The Spider valve, I expected to hate, tried it, dug the shit out of the clean channel, and was so excited that after 15 minutes I started flipping through the gain channels thinking maybe they'd finally got something right. Nope, they sucked.
Thebigcheese - that's a LOT of different tones you're after, from a single amp. Rage Against the Machine is about as Marshall (800-ish) as you can get, while a whole bunch of those other guys are very Recto of Peavey 5150 in nature. And, Lamb of God is the quintessential (well, quintessential
modern, at least - let's not forget early Metallica) Mesa Mark-IV sound.
If I were you, I'd spend some time thinking about what exactly that elusive sound in your head is, because the difference between "Down Rodeo" and "My Curse" is pretty night in day. Reading your later posts, it sounds like you're gradually starting to, and if you're after...
Overall, I've noticed that I really like the sparkly clean tones and crunchy (but still smooth), heavy distortion. I don't mind if the cleans are a little "dry"--I actually kind of like that.
...then here would be my recommendations:
Peavey 5150. You can grab the heads for cheap, it's not uncommon to see them around $500-600. The clean channel is, um, not that clean, but for metal rhythm these things slay, and as long as you don't mind a "dry" clean sound and don't have to footswitch back and forth between that and a gain tone while gigging, they're tough to beat for the price.
Mesa Boogie F-30 You'll have to find this used, as this was the forerunner to the (far inferior, IMO) Express series. One of the best cleans I've ever heard from a Mesa (or anything, really), and a great, great gain channel - not the most flexable EQ section, but the gain sound it produces both slays for rhythm and is a surprisingly nice lead voice. Fairly aggressive, but these things are just great amps.
Mesa Mark series, used. Now that the Mark-V is on the market, you might be able to snag a Mark-IV head or combo for less than a grand. This is pretty much Lamb of God in a box, thick, saturated, and smooth. Also worth a look, on the cheap - the Rocket series, but these will be very hard to find - I used to own
a Rocket-44, and it's a very Mark-like gain sound, but without a lot of the options.