What could I do to upgrade current system for $300

What would you guys recommend I get? I have around $300 right now that I want to spend to upgrade my current system. The system that I have right now is a Boss HU, with subwoofer and preamp outputs, and four Boss speakers(2 6x9's and 2 5.25's). I know that it isn't the best quality setup, but I haven't had any problems with it so far. All I want to do is put a subwoofer in my trunk to give me more low end, without screwing around with the four speakers I already have. If its possible when/if you respond could you put a link to the parts that you recommend are the best bang for the buck that I will need, and have the setup not exceed $300. Also I know that Boss is not the best when it comes to quality, but they haven't given me a problem and actually sound good, I just want a little bit more bottom end. You guys don't have to keep reminding me about my mistake.

its obvious you don't know good companyies, becuase boss sucks ass. i would recomend you upgrade your head unit, and your speakers, get a clarion and then a nice set of component speakers, i think you will be suprised at the increase in the low end.

I know Boss isn't the best quality by far, but the speakers that I have now aren't giving me problems and actually have very good low end ( I know what sounds good and what doesn't, even though I don't know the best companies), all I want is an additional boost in low end. I don't what to spend the money right now on all new speakers and head unit especially since they aren't giving me problems.

I was looking at this setup: http://www.crutchfield.com/S-5KeWHeqBLI4/cgi-bin/ProdView.asp?s=0&c=10&g=183650&I=700FREEBOX&o=v&a=1&cc=01&avf=Y

I know that crutchfield is a little expensive but this looks like a simple way to do what I want to do. One question, I am stuck on what amp I need to get to run this setup. The description says that the enclosure only has one speaker terminal and the subs need to be wired together, what amp will be compatible with this that is under $180.

dude.... boss sucks crap.. downtown they sell their 10 inch subwoofers for like $10 a piece and they say they are 400 watts, my friend bought it, u know what happen? my friend put 250rms thru it and blew the sub in a day.... as for the head unit, jus throw it away, get a clarion / alpine / pioneer / kenwood / panasonic or something around that range a cd one will cost from $100-$200 a mp3 one will cost $150-400 depending on the brand

Go with an ED e12K.44.(single4ohm)
http://www.edesignaudio.com/ep2/12k.htm
powered with
http://www.avionixxusa.com/ep/axa_4002.htm

OR

Adire Shiva
http://www.adireaudio.com/mobile_audio/drivers/shiva.htm
powered with
hifonics amp (ohms are specified but im sure its 600@4ohm)
http://www.sounddomain.com/sku/HIFNX600
Both a little over 300 but are good subs and amps...they are sq subs but either would pound.

Where could I find a prefabed enclosure for the ED e12K.44.
That is probably the one that I will go with.

I also know that Boss sucks and I am going to be replacing everything soon.