OK, Wagner has been recommended, but be sure to get a compilation of his orchestral tracks... I especially love the "Tannhäuser Ouvertüre".
Another great and a bit similiar composer would be Richard Strauss.
The Intro to his "Also sprach Zarathustra" is very well known, i think it is played in a science fiction movie - this one is very metal.
Edvard Grieg has some nice stuff... but i there is also enough of him i do not like..
I also like Beethoven, but all this older stuff, despite being musically diversed and very good and so on.... it is a bit tame compared to the newer composers, though there are epic and blasting moments.