Slash's distortion is due to the volume, not the gain. I'm not a tech head, but I know there are two sets of valves in an amp, and if you get the power ones to distort (not the preamp ones), your sound will be much smoother... I know that's what Angus Young does too.
Also, the guitar has a large part to play in the sustain... Les pauls are known to "play, go grab a bite, it's still playing" as our friend from Spinal TAP puts it...