My thoughts on the TG2 on guitar cabs, FWIW...
1.) If you have a Hi-gain head, such as a a dimed Mesa Triple Rectumfryer, it can be problematic... the TG2 can tend to be "fizzy" on the top end, and if you don't cut back on the gain on the amp head, depending on what mic you're using, you can end up with a big wall of "fizz" - IOW, very excited-sounding guitar tracks... this can work sometimes, of course, but that's one of those things that you can't get rid of once you track it. "Fizz" is unremoveable.
2.) I happen to think on guitar cabs the TG2 works better on darker or more mid-forward mics such as the -57 or Heil PR-30... I don't think it's very friendly in that application towards something like the Audix i5 or Sennehiser 609, for example.
3.) It's a "Warm & Wooly"-sounding preamp... which is fine if that's what you're looking for, but if you're wanting something more of that "fast-snap" sound, it may not be what you really want... IOW, the TG2 can be a good, "thick"-sounding pre, but if you want fast snap & attack, it might not be the way to go. As a result, it's not the best preamp for certain types of layering of guitar tracks. It's got a little bit of lo-mid "mushiness", and if you layer too much "mush", then you get... well too much mush. lol
James, I think it all depends on what YOU consider to be "heavy" guitars, and also what you plan on micing them with... If you want a single or double-tracked, thick guitar tone, then the TG2 might work for you - if you want something like multi-layered "MEGA-CRUNCH/CRUSH" guitar traks, then I tend to think you're looking in the wrong direction.