I know this very ancient thread, I know this is a very ancient topic, but I can't resist and I want to answer, because I am a fan of metal music
Well, the simplest way to go would be to get just about any head unit with pre-amp outputs (since the head unit alone won't do you much good if you're wanting power and volume). Find a decent 200-400 watt amp (prefferably 4-channel) then...
1.) Install better door speakers. You can either use the standard-size 6.5" speakers which gives you a considerable number of options. Or you can try upping the size a bit. I've had 6x9's in my doors for about a year now and love 'em. Another guy on the board has 8"s in his.
2.) Forget the headrest speakers. Either install a pair of 8's in the rear deck (full instructions are located in the Garage's Audio section). Or you can take the quick-and-dirty route like I did and buy a pair of small 6x9 boxes (yes, I have a thing for these speakers) from any audio store (I just bought mine at Wal-Mart; very inexpensive route), buy a pair of speakers with high power handling, hook them to the amp, and you're all set!
With my current setup I have a pair of Pioneer something like this
https://soundrating.com/best-12-inch-subwoofer/ (can't remember the number...) 6x9's in the doors (the trick to using these is NOT to have a set with large magnets or they'll interfere with the door hinge). And just this weekend I got my hands on a pair of Audiobahn AS69Q's which just blew me away once I got them hooked up. 95db sensitivity, 200w RMS power handling, and they're just about the nicest-looking and cleanest sounding speakers I've ever heard.
Oh yeah, and they're LOUD. I listen to everything from Techno to Grunge and Industrial myself, and it sounds incredible. Just don't get yourself in trouble.