I think those amps need a Tube Screamer-type boost and tighter-sounding pickups to do any kind of Metal that isn't Doom or Sludge. Not because they lack gain, but because, like you said, they're super fizzy and loose otherwise.
But yeah, like others have said, if you're not into scoopy kinda thick sounds, then the TH30 is probably not for you. Especially because the usable range of that shape knob is very limited, honestly. I'd say below noon, it sounds like there's a blanket over the speakers, and above like 1:30-2:00, it sounds like a caricature of a Metal amp or kinda like a Big Muff.
I've played one, and I also had the Dark Terror which is kinda based around the same design.
I don't think they're unusable, though. But they definitely need all the help they can get.