Mh, band sounds promising. I'm momently on a roll about doom metal and retro rock and alternative stuff, for example recently I discovered Avatarium and I'm digging them a lot, especially the first album.
Wikipedia says this:
Cult of Luna's sound has progressed from early material being heavily doom metal influenced to one much less aggressive and more concerned with orchestration. Fans and critics have termed this sound post-metal. The band is considered to be at the forefront of the genre, along with contemporary proponents Neurosis and Isis.
For someone who currently enjoys some doom (see Avatarium's first album, the self titled, or maybe Candlemass' Psalm for the Dead... I know, I know, Messiah Marcolin made history with the band but I stumbled upon that one and I like it), but loves also orchestrations and enjoys almost exclusively clean vocals (no growls, definitively no screams), which album would be a good starting point?