Unfortunaely, when you say you are going for strictly sound quality, you're looking at investing your money into the best set of spaekers, you can get, and the best source unit. That would take your $750 by itself. Now, that said, you
could buy used, and some items from
ebay. What is hard for me to know without talking with you at length, is what you are willing to sacrifice on. Personally, with what you said you want to do, I would run a sealed enclosure for the sub. You can get the bass sound through the rear deck by removing the rear speakers completely. Or, just fold down your rear reat to expose the sub to the passenger area. You might have other ideas, but I would not want to run a sub in IB. In fact, there are very few manufacturers that suggest running IB, at all. Besides, if you can make a ham sandwich, you can build a sealed enclosure. It's not hard, at all.
For the amp, I would get a 4 channel, that you could bridge a channel pair for the sub, and run the other two to your comps in front, passive. If you get a good set, you will not need the 3 1/2's in the dash.
There are lots of opions for what you want to do, but for what you're looking to spend I would go with a set of MB Quart RVF-216, for $159. A Cadence Amadeus M4000 amp for less than $200
http://http://cgi.ebay.com/CADENCE-AMADEUS-MODEL-M4000-4-CHANNEL-AMPLIFIER_W0QQitemZ290210560785QQihZ019QQcategoryZ39739QQssPageNameZWDVWQQrdZ1QQcmdZViewItem, and a deck for $125- $200. Don't skimp on the speakers, or the deck. You should try for better than 75 W/channel on the amp, and I'll just recommend a Boston G2 12" sub. It fits in a 1 cft sealed enclosure, and you can get one off
ebay, NIB, for less than $100. There are others, of course. These are just some that come to mind that will fit your needs nicely.