it's really a good
tube amp (compression) getting hit with a DS-1 (extra gain) turned up really loudly (sheer volume).
Also worht mentioning- Satch has said in interviews that on live versions of that he tends to pick in time with his vibrato to hide the pick attack a bit to help the sustain a lot.
and while this is hardly satch-approved, generally if you raise your action a bit, sustain improves.
-D