Sub and Amp?

The larger you go on a sub, the larger the amp, the larger the box is a good general rule of thumb. If you only have the stock infinity system, you do not want to have bass overkill. You may want to consider two of the subs, in the 8 ohm version and the Strike amp, model # S2.600.2. Two subs tend to smooth out the bass response. It takes the strain off of the one sub to hit all the lows and give good loud volumes as well. Two subs also are going to play louder and deeper. More power will also allow it to play louder, you could also go the route of dual 12" for louder and deeper yet. There is really no end to it. If you purchae locally from a good shop, make a deal with them to change the amp if you need more power or to add a second sub if it is not quite enough. This is where the local dealer will be able to help you more than any other.

I'm going to add this to my list of memorable quotes...

With two subs there is also an effect known as mutual coupling, another reason as to why people place subs in close proximity to each other. Not only does it play louder, but two drivers working together have a different effect on response. You are correct with the assumption that they can both play the same deep frequencies, however two drivers playing the same frequencies are now reinforcing each other for the full range of frequencies being played. Two drivers operating at the same frequency no doubt have more output and hence will play (lack of proper words) the same low frequencies, they will just be played with more authority, thus the perception of deeper bass. I should have written that a little different, sorry for the confusion.

I am still fairly new to this forum but I'm going to put in my 2 cents.  I wanted the same thing you did, nice clean and crisp sound with hard hitting tight bass when you want it.  I also am on a budget like most of us and also don't want to rattle the body off the frames of most cars around me, including mine.  I choose the Infinity Perfect 10.1 sub and the Blaupunkt PA2100 amp.  The sub can handle 350 wrms and the Blaupunkt amp can produce 300 wrms bridged.  The amp is pretty compact and does not get as hot as the older designs.  If you choose this sub make sure you purchase the Perfect 10.1.  Infinity makes several different versions of the 10" sub but only a few that can handle 350 wrms.   

I will probably catch some flak from some of the guys on this forum for recommending the Blaupunkt but from my experience it worked very well.  Good luck.