Ok, after quite a long search this morning I came up empty on much information on the b5 s4 AUDIO system. First of all, my car is a 2001 S4 and a few of the speakers are starting to rattle and come loose. I would like to completely replace the Audio system with a 7" Double DIN Navi/DVD/CD/MP3 etc unit. What are some good models that will fit? What wiring/mounting kit do I need for a clean install? Lastly, I believe that the speakers are 5 1/4 with a 1 inch tweeter in the front door, 5 1/4 in the rear doors, and 6.5 in the rear deck. Is this correct?

 

either kenwood or alpine will do just fine you also need a amp integration wiring harness best buy knows a lot but they dont carry it so got to installer.com for the harness and then have best buy hook ya up for speakers prob alpine type s are the best for sound

 

You shouldn't have to replace the amp just if the speakers blow, just the speakers themselves. I still have my original bose spaekers but wouldn't be able to get to them for a few months (cuz they are in storage til I buy a house)if you really wanted to just replace them with speakers that will blow again.
Like most things audi, it can get expensive. I had a couple bose blow and decided to just replace them all. There are kits to make bose speakers work with aftermarket stereo (which is what I did at first plus an amp and 12" sub)but don't know about making your stock unit work with aftermarket speakers--guess anything's possible. The problem is the impedence of the bose speakers is 1 ohm and most aftermarket speakers are 4 ohm and most aftermarket stereos won't do 1 ohm either. If you want to just replace the speakers, you can do so and just connect them to a new aftermarket amp (cuz bose has a separate small amp for each speaker usually)and then get the bose kit/wiring harness to make them work with the stock radio OR just replace everything like I did and not have to worry bout any kind of compatability issues if they should arise.
Whatever you decide to do, I went with Focal speakers. They are high end and sound AWESOME and can handle way more power than they say they are rated for without having to worry bout them blowing. There are several different series and price ranges. I wanted sound quality over loudness and ended up getting both. They are crystal clear with any music and though were a little pricey, were worth EVERY penny! I'll never buy anything else again!
A couple good alternatives to the Focals would be the Infinity Kappa series, which are definitely less expensive but were a little bright for me but good none the less, or some of the new JL Audio components. Best bet is to go to a car audio place and just listen for yourself and see what you like. BTW, i also got a double DIN Kenwood(which I've always had great luck with) head unit that plays dvds, etc and looks great in the factory spot.
I was a stereohead before an audi fan so feel free to msg me if y'all have any questions.
ADDENDUM: Just went to have my kenwood pulled out(sending off to get couple buttons fixed before the warranty is up) and the factory put back in. The factory WILL NOT WORK with aftermarket amps. Has to do with lack of connections on factory unit and possibly Ohm loads but maybe not. They instead were able to hook up a cpl Yconnectors to all my indash wiring and now my whole system is temporarily rinning off the earphone jack on my iPOD!! Sounds great too!!!