I was listening to some Haggard just now and I was inspired to broaden my musical horizons to some classical music. Anybody have any suggestions as to which composers I should check out first? I generally prefer classical music which uses a lot of low-sounding (double bass and the like) and brass instruments. I also listen mostly to metal and I know some of the musicians use classical music influences so does anybody know which classical composers were the most influential?
Bach is my favorite, lots of triad arpeggios in his music and such... Bach = Metal before metal. There are tons and tons of great composers though. I'll try and compile a better list for you if I have time today, but start off with some Bach.
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The influence of Bach is huge, Haydn, Satie and Rachmaninoff are also very noteworthy (as well as the ones ernest listed of course!).

Since there's quite the abundance of works by these composers, I thought I'd recommend a few based on what you said:

Bach - Clavier concertos no. 1 and 7 - personally I always found these more enjoyable on the piano rather than on the cembalo, historical accuracy be damned .

Vivaldi - Violin Concerto no. 2 op. 8 (summer)

Grieg - Piano concerto in A minor

Naturally you should take a listen-through to Beethoven's fifth symphony.

Also, if you´re a fan of piano solo music, check out some Bach suites, rachmaninoff's Morceaux de fantaisies (op. 3) and of course the whole of Beethoven's 14th piano sonata ("moonlight sonata", though he didn't name it) - note that listening to merely the first movement leaves out the climax!

Haha, a rather populistic list but hopefully these works will give you a taste for blood
Uh, if youre looking for low sounding i would def. go with wagner.

Also, if youre looking to expand your horizons on country music, I can suggest a lot of good stuff for you in that area.
When they kick at your front door
how are you gonna come
with your hands on your head
or on the trigger of your gun
Quote by Gris-Gris-Man
The Rite of Spring - Igor Stravinsky

This is so true. Also, The Firebird. And while you're at it: Pictures at an Exhibition by Modest Mussorgsky, Partiels by Gerard Grisey, Symphonie Fantastique by Berlioz, all of the minor key Beethoven piano sonatas and symphonies, 3rd Piano Concerto by Rachmaninoff (the most shredtastic of all concertos), Mozart's Requiem, Verdi's Requiem, and John Dowland songs such as "Flow My Tears" or "In Darkness Let Me Dwell" for those super-depressio moments.

And for opposite day try: Prelude to the Afternoon of a Faun by Debussy, anything by Ravel, Missa Prolationum by Jan van Ockeghem, anything by Josquin dePrez, anything by Adrien le Roy, and the sexiest piece EVAR: Tabula Rasa by Arvo Part.

Hopefully this and youtube help you on your journey.
Dmitry Shostakovich, Symphonies 5 through 8
Any works for solo piano by Rachmaninoff, Chopin, or Schumann.
The Rite of Spring by Igor Stravinsky
Other suggested composers to peruse: Dvorak, Tschaikovsky, Rachmaninoff's choral and symphonic works

Late era Romantic work, in my opinion, is the best. Baroque era music like Bach, Vivaldi, etc. I find very boring. Classical era has its gems, Mozart (obviously), Haydn. Beethoven straddles classical and romantic, and his works are very good
I quite enjoy Debussy, his piano peices are cool.
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