Tone is a very subjective matter. What doesn't sound good to you might sound fine to me and tons of others using the same gear. Words like "glassy", "harsh", and "thin" only go so far in describing what you're hearing. You might want to consider posting an audio clip of what you're hearing, and maybe even an example of the kind of tone you're looking for, like a YouTube video or something.
Anyway, beyond that, "high-gain" to me starts with the Mesa Boogie Mark IV Lead Channel model. Both Gains at full, treble at full (treble full on a Mark IV will almost negate the Mid and Bass controls, so I wouldn't worry too much where they're set). That will start to give you tight palm muting, but if you want it tighter you can put a Tube Screamer model in front of the amp, Gain 0, Tone at 5, Volume full. Then use the built-in parametric EQ on the Mark IV to beef up the low end and maybe scoop out some mids. That's a good "basic" high-gain "thrash metal" tone to me. Might not be anywhere near what you're looking for. No way to tell unless you post clips. There are a number of other good high-gain models also.