I think they get the full sound out of the thrashed root note riffs by double-tracking the part, getting the bass to mirror the guitar riff, recording with enormous overdriven stacks, using specialised high-gain pickups on their guitars, compressing the hell out of the recorded parts... et cetera! When you're a band like that, you want to be as loud as possible - I think it's more about turning everything up than having a full-bodied raw guitar/amp tone. The guys from Killswitch play with comparatively thin-bodied Caparison axes, and they nonetheless get that in-your-face sound.

As for the technique - I spent a few months bringing my metal rhythm skills up to scratch, and you probably don't want to hear this, but there is no shortcut to playing riffs like that - you've just got to pick your example riffs, start slow, and build them up to speed. Pick a riff for each rhythm technique you wish to work on - eg. one that's got "galloping" triplets in it, one that's got 16th-note speed picking, and any other techniques you wish to work on. For those two techniques, I used "Angel of Death" by Slayer (you don't have to actually LIKE the song :p) and "She-Wolf" by Megadeth. I'm not too familiar with the All That Remains and Killswitch Engage catalogues, but you ought to be able to find a couple of songs that fit your needs.

Just a little tip about 16th-note speed picking: if you're playing a riff like this that isn't palm-muted, you might be able to get a little more fluency by moving your hand back from the strings and picking closer to the bridge.