I would run the stock rear speakers off the H/U like I did and spend the extra money on everything else. I spent about 1k on just equipment.
Here's my setup. http://www.cardomain.com/ride/3153500
Sloppy bass- That box is a little on the large side. Website says 3 cubes.
Lack of midbass- Make sure the tweeter attenuation is all the way down.
Hissing- Getting a better amp for the comps would probably help.