So, my idea of a “good metal tone” and yours are a little different, probably…
…but, generally, I can make a couple comments. First, they’re probably NOT that scooped - the guitar is a midrange-heavy instrument, so that’s where it’s going to fill the most space in a mix. “Classic” metal tones were generally a “everything on 10” Marshall partly because that helped overdrive the amp further than it otherwise would have. I forget what Megadeth ran through, but Metallica famously used Mark IIC+ heads with the mids boosted but with a V in the graphic EQ to cut them back (which, to be fair, is still a fairly mid-heavy sound, considering how much midrange the Marks have) run into a Marshall poweramp.
You can scoop rhythm guitars more in a recording than you can live… But generally the more you scoop the “smaller” they sound in the mix as they tend to get buried with bass and drums. Gain has a similar effect - as an amp overdrives it seriously compresses the guitar sound, so you tend to smash transients down further and further the more it comes up. You can go too far in the other direction by backing off the gain, but in the early thrash days you tended to see a bit less preamp gain and guys just picking a lot harder to compensate than you see today, where I could push my Mark-V into seemingly endless sustain and compress it to the point where the lightest pickstroke imaginable is enough to riff on. It just would be a flubby, overcompressed, dynamicless mess by that point. 
The “not sounding thin” probably has a lot more to do with the bass guitar than you think - a heavy rhythm guitar tone rarely is super deep, but rather lets the bass support the fundamental.
In the mix… High-passing is fairly common in mixing heavier guitars, anywhere from starting right around the fundamental (80hz for a low E, and IIRC around 60 for a seven string low B), to potentially a fair amount higher than that, 100-180hz or so. I also tend to low-pass, low enough to clear out the high end fuzz, not so low as to be audible under cymbals. Sweeping a filter between 6.5khz and 8khz or so and listening until it sounds right, and if you want running a fairly narrow Q can give you a little bit of a boost at the cutoff point, for a little more aggression. I’ll also usually pull back a couple dB in the 650-700hz range in rhythm guitars.
But, a clear, punchy, saturated but not squashed guitar tone with adequate but not massive low end, double tracked, and with a tight bass performance supporting the low end below it should get you in the right ballpark.
For kicks, it might be worth digging around YouTube to see if you can find any solo’d rhythm gutiar performances from Megadeth or Master of Puppets era Metallica, to steer you in the right direction for what a thrash rhythm tone sounds like isolated.
