Blah. Recommendations about x 10" drivers etc. aren't very helpful in my opinion. He needs to determine how much space he can dedicate to subwoofers and buy the right subs for the enclosure he can fit. Who gives a crap if it is a 10" or a 12"? Let your goals and reality of your installation environment drive your purchases... not some "x" woofer diameter.
Metallicaudio: Learn what gain actually is and how to set it correctly. A statement like "half cranked on the gain" tells me that you didn't bother learning how to use your equipment. I don't like Audiobahn either, but Audiobahn isn't at fault here. You are. Learn to set gain correctly. Also, if the amplifier has a constant draw even when the car is off, then you have installation issues. Trouble shoot that or take it to an installer.
Here's my take on your situation. I used to listen to metal. Most metal, especially 80's metal, doesn't have much deep bass. You'll want very capable midbass and hot high bass 60-120hz. You made a few mistakes in planning your system. You bought a much larger amplifier than you need. An amplifier like that doesn't make your subwoofer hotter with respect to your mids and highs. Let me repeat that, power ratings do not make your bass hotter compared to your mids and highs. A 10000000 watt bass amplifier will still disappoint unless you set and plan your system correctly. My advice:
First sell the stupid 1800w amplifier. It will be way too much power for the ported system I think you want... and your electrical system.
1) Find and buy a subwoofer with good extension from 100-200hz. We'll cross over at 100hz for you. It'll add some hot kick I doubt you'll get with whatever your front stage is. This xo point isn't ideal, but it will probably make you happy.
2) Find a sub that works ported in the space you have available. For you, we want to work on increasing efficiency. Try to get multiple woofers, if you can. Making your 60-100hz bass efficient is going to be more important for you than very low extension.
3) Buy a 4-channel amplifier with high pass crossover for your mains. You'll need it -and- it will make integrating your subwoofer amplifier easier. 50x4 is plenty.
4) Buy some midbass capable mains. Do a quality install up front. Same for whatever you put in the back. A lazy install will kill midbass. High pass the fronts and rears at 100hz. Don't set the gains yet. Turn them all the way down.
5) Now, Set the gains on the SUBWOOFER amplifier CORRECTLY. Now. Leave this gain control alone. Never touch it again. Leave the bass boost off. Never touch it. Never turn the volume knob on your deck past where it was when you set gains. Do these things and you shouldn't blow anymore subs.
6) Finally, set the gains on the main amplifier. This is where you can give your system hot bass. Make the gain lower on your mains than your subwoofer. This will make the subwoofer seem HOT and loud relative to your mains. This was what you were trying to accomplish by turning up your subwoofer. This is the right way to do it. Repeat: don't turn -up- the subwoofer past where it should be; turn the mains down.
New Project: 2003 Pathfinder