Jonno said:

The preferred source for this mix is an earlier generation laserdisc (since aside from lossy compression the track has also had some 'improvements' made since 1999). I favour the 1992 collector's edition but there are others.

And I think I can help with that. I have this LD:

http://www.lddb.com/laserdisc/10590/1090-80/Alien-(1979)

which I believe is the first laserdisc to have the soundtrack in digital PCM

and Dolby Stereo. So it would be closest to the original source.