I'm looking for a real brutal thrash/death metal sound in the vein of Beneath The Remains era Sepultura. Is the RX120DHS Half Stack able to pull of that sort of sound?

If not, what affordable half stack do you recommend?

I'm trying to work in the $500-$750 range.

Go second hand. You got your Crate blue voodoo's, Peavey Ultra's. If your willing to go a combo, Marshall JCM's will nail that tone.