Well, I know from my own experience, when I do deep growls my throat vibrates/growls 2 differnt ways. One is a more rounded out throat vibration (almost impossible to describe), and one is a very sharp, brutal growl (comparable to Mikael Akerfeldt's work on Bloodbath). The first one is very easy to do. It can be done very softle, sitting down, whenever you want it. But it sounds like crap. To get the second one, you have to growl a lot louder. You also really have to clench your stomach, and try to use your entire throat, all the way down to your lungs, to growl. Even then, it's pretty hard. It gets a bit easier if you combine the 2 types of growling. Then is sounds sorta like Mikael's work for Opeth. If you pactice enough, you'll get what I mean. It also helps to have a naturally deep voice (which I defenitely do).
I also know how to growl like early Swano, Morbid Angel, or Dismember - but I'll save that for another post.
For high growling/screaming like Amon Amarth (or Dark Tranquility, or Ensiferum, etc), it's pretty easy. Get your voice into a high note - not like a falsetto, but high (like the opening scream of Slayer's Angel of Death) and then distort it. It's a very different growl than the previous 2, so just try and mess around a bit. Try to imagine it in your head how the DT/Amon Amarth singers do it. Then try to emulate that. It shouldnt be a problem even if you have a deep voice (my voice is deeper than most grown men's voices, and I'm only 16).
To sound like Maniac from Mayhem, try to go as loud as you can as loud as you can. Try toind a falsetto really, really loud. At a certain level, your voice should just distort itself. Once you get it, try going a bit quieter, you don't wanna damage your vocal chords.
To sound like Dead from Mayhem, just yell as loud as you can without going high. Don't hold back, just yell as loud as possible. For me, that makes my voice distort in the same way Dead's did. Try adding a "sharp" or "metallic" tone to it, by forcing it a bit through the deeper parts of your throat.
And that just about covers it. Excuse the typos, it's 1:30 AM here, and I've been up since 6:00 AM.