Guitars will have a lot to do with sustain, and also a healthy touch of good old compression. Compression will help clean up your tone and give you more sustain without needing to up the gain. Guitarists always use a lot less gain than most people realise though. Also, Slash would be using an overdriven sound got by cranking the amp volume rather than the gain, as mentioned earlier. This is very hard to do if you don't want to kill your eardrums and you don't have a recording studio however. We just have to live with it. Or make an amp like Lordathestrings (by the way, how is that going?)