What size are the rear deck speakers on a 00 S4 with bose, also how much work would it be to fit 6 by 9's in that space?? Thanks A lot.

~ Jason

They're 6.5". You may have to do some minor cutting to get a 6x9 in there, but why? A 6.5" woofer will drop in nicely and will give you better sound wave quality because the wave is round instead of elliptical.

I know this is a little crazy, what about 8" subs in that space, how would that work?? And then replace the rear door speakers with 5.25 components. how would that sound??

~ Jason

Well, you could definitely do it, if you're willing to permanently modify the rear deck. As for the rear door, I would still go for the 6.5" for midbass support. A 5.25" can go in the front door along with the separate tweets. View your car as a concert venue . . .where do you want your highs, mids, and bass to come from during the performance? You're always in the front, so you want clean sound in the front. You can go ahead with the plan for the 8", but be prepared for some serious cutting and sound dampening material (Dynamat) for your rear deck. Good Luck!