high quality deep bass

I have a Scion xB and try as i might, I cannot get the deep bass I was able to get in my Camry (I could get 20hz at good volume).  I realize this is due primarily to my not having a trunk, but I'm still hopeful there's something I can do.  Right now I have a Soundstream RFW-12 sub (600w rms).  It sounds very good, it plays very clean, but it won't reproduce bass at the depth I'm looking for.  I know it's not the subs fault bc I've tried numerous subs (audiobahn, mtx, kicker, infinity) with no luck.  I've also tried different sizes and setups ranging from one ten, to two tens, to one 12 and nothing really truly gives me what I'm looking for-although one twelve seems the closest.  Please note: I'm not JUST looking for deep bass I also want accurate bass so I'm really only looking for some sort of sealed enclosure.  Also, i want attentution of the higher frequencies (over 80hz).  That is to say, I know I could probably throw in a couple of twelves and get deep bass but I'd likely be subjected to an overwhelming amount of non-deep bass as well.  So, in short, I want some sort of setup that will give me clean, deep bass with a minimal amount of non-deep bass--any help would be grealy appreciated. 

1 last thing.  Most 12" subs recommend sealed enclosures with around one cubic foot of air space.  I know the kicker solobaric L5 can push up to 2 cubic feet.  Do you think going with a sub that can move more air might give me what I'm looking for?

The enclosure is a big proponent of what kind of extention you will get on both ends as many have already mentioned.  But the sub will effect this as well.  If you want lower frequency extention, then excursion is a very good thing to have.  This is because for every halving of frequency the excursion must increase by a factor of 4 to keep the same output level. 

You also mentioned that upper bass frequencies were too loud.  This could of course be caused by a peak in the response.  However it may also be from harmonic distortion.  Keep in mind that this type of distortion shows up as peaks one, two and even three octaves above the fundamental frequency.  For instance if your sub has horrible second and third order distortion then peaks will occur on top of the natural harmonics for say an instrument with a 30 Hz fundamental at 60 and 90 Hz.  And this is something that can't be processed out without effecting the fundamental frequency directly. 

It sounds to me what you need is a high excursion, low distortion driver.  As a suggestion you may want to look at JL's W7.  It is rather spendy, but there are few other drivers on the market that can match it's performance.  Here's another possibility for you: http://www.acoustic-visions.com/~acoustic/products/subwoofer_drivers/CSS_SDX15/  The down side to that sub is that so far it is only offered in a 15 (the 12's should come in the future) and it does require a large enclosure compared to the W7.