output volume reflects mastering standards of the day. the real issue at hand is clarity and frequency separation. the final mixdowns of the first four albums actually hurt the quality more than necessary. have a listen to all singles from 2pacalypse through thug life and you'll find that singles and promos of those albums hold tremendously better sound quality. by remastering and re-mixing the songs, they'll never be the digital sound of all eyez, but there will be much more definition in the highs and mids.
all eyez could be remastered too... and i mean better than they have even with the dual discs... there's a lot of feedback from bad wires or electrical flow through the instruments of the date in there, and while it does give the album character, i think that it was mainly meant to have a very digital sound.