Metallica's heavy rock and interesting lyrics have been influenced by classical music. My album suggestions :
Black (1991) - a real turning point in their music
S & M (1999) - Symphony & Metallica, recorded live in Berkeley California on April 21-22, 1999 with the San Francisco Symphony. The SFS really takes the rough edges out of Metallica's music while enhancing it at the same time.
SFS conductor Michael Kamen passed away in 2003. Check out his article in the New York Times from 1972 on rock music, classical music, and progressiveness.
http://www.michaelkamen.com/news.php..._from=&ucat=2&