Updating my Sound System (On a budget)

[Newlin]

So I've just recently acquired my Alero and the guy who had it previously took the liberty of installing an aftermarket sound system. It is alright but he had three 12" subs in the back which literally took up all the trunk space and made getting to the spare impossible. I've since taken those out and have been running on the stock speakers which sound inadequate and thin.

First, here is what I have to work with. I have a power cable running from the battery to the trunk along the drivers side and the power on/off (blue cable) as well as the RCA cables running along the passengers side.

As for the equipment that was inherited with the car I have this head unit:
http://www.crutchfield.com/S-zFY8OVd...H-P7700MP.html

I have 3 12" MTX subs 4 ohms 200W (I can't find the model number but here's a picture):
https://www.dropbox.com/s/ecany4dzxl...2008.33.47.jpg

And I have this amp to work with as well:
http://www.qualitycaraudio.com/store....asp?sku=LA768

I'd like to avoid using the subs just because I like all the trunk space and purchasing a pair of smaller enclosures makes this all more costly. With that said, here is what I would like to do, let me know what you guys think.

Head Unit: http://www.amazon.com/Dual-XDMA6415-...I3N17A5FDQ7N71

I mainly want this for the AUX in, USB port, and the Bluetooth for playing music and taking calls. My current unit supplies no method for inputting my own music outside of a CD.

If I go the route of no subs I'd like to get a set of Infinity Reference speakers to replace the current ones (or something of similar quality) and perhaps keep the amp to provide them more power? (I'm still pretty green on audio)

If I end up keeping a pair of the subs I have now I'd probably end up getting a pair of these enclosures for the back:
http://www.amazon.com/gp/product/B00...o_cart_title_1

I've also noticed that I only get a couple of radio stations in clearly and my thought is that when the head unit was installed the antenna amplifier was not powered, it could be something else though.

In the end I'm leaning more towards just upgrading the stock speakers, so if you have any additional suggestions there that'd be great.