#1 There are a ton of companies that can match and/or exceed JL in SQ and SPL. They are very good SQ subs, I would never knock them, however, you can achieve the same results with another companies drivers.
RE, http://www.bladeice.com/prore.htm
DD, http://www.ddaudio.com/dd/default.asp
Adire, http://www.adireaudio.com/
Kicker, http://www.kicker.com/Main.cfm?CFID=...TOKEN=34550419
Check here for more, http://audioforum.termpro.com/cgi-bi...ultimatebb.cgi

#2 JL offers many different lines of drivers based upon price and level of output, powere handling, and application. The best thing you can ever do is trust your ears. Go find someone that has a pair of twelves and have a listen. They don't all have to be JL either. Listen to several brands. You may like something else better?
As far as amps. Pick this item first, You can usually order your selected subs voice coils to work with your selected amps impedence level. It is more difficult to to match an amp to a sub, however, not impossible.

#3 One option is to face the subs through the back seats. Then seal the trunk off fron the cabin to prevent sound waves from beating your trunk apart. This will eliminate 90% of your rattling when done correctly. Dynomat is a good option if you use a more traditional method of mounting and facing your subs.

#4 http://forums.maxima.org/showthread.php?t=343247

As far as the install, it depends on how finished of an install you want. You will pay more for a more than basic install. All shops are different and have installers with different capabilities. The sky is the limit in this area.

I know a very good installer in Jacksonville. He will be doing my work. www.loganscarstereo.com There are a bunch of others throughout the state. Pick one based upon thier installs, not what they tell you they can do. When you check out Logan's site, look at his yellow hummer H2. He did that install. It is JL sponsered as well. This is the caliber of installation I look for in a shop. And yes I will be driving all the way up there b/c I trust him.

Good luck.
Chris

 

Honestly, I would find a good reputable shop that you can trust isn't trying to rip you off. After you find a good shop than decide on the gear. There are plenty of alternatives to JL but unfortunately not many places carry them.