Which are the correct frequencies to improve guitars / drums / death vocals / etc?

Assuming flat frequency response by your sound equipment, use 0dB across the spectrum. Precisely the way the mastering engineer intended it to sound.

What kind of equipment are you using when you listen to this music? I would say it really depends on your equipment, and your ears/hearing. I would play with the EQ some to see what sounds best to you.

I know most EQ presets I have seen for "rock" type music generally have an increase in the lowend up to about 250~300Hz. Most of the vocal frequencies (not sure the exact range 300~1000Hz for prominent tones?) are usually lowered a bit and the higher frequency ranges tend to have an increase similar to the lower frequencies. I hope that makes sense.

When I used to fiddle with EQ a lot I almost never used the same EQ on different equipment. Use whatever sounds best on your current equipment. I gave up with the fiddling, I'm happy with the standard sound quality of all of my equipment without any tampering. Good luck!

What equiptment? Hmmm.... Mainly my Creative Nomad player, but also the PC (board soundcard...) and my father's Hi-Fi (this is a good one).

I played with the graphic EQ but I didnt get (yet...) a real "improvement"

Post EQ should be used minimally, assuming you got good quality recordings from the instruments (tuned drums, good mics and mic placement, high quality guitars and amps). Generally, a little compression on the snare drum and guitars can enhance the sound as well. When testing the downmix, as said before, it is best to judge the sound flat (no eq) on your sound system.