8p sub blown there must be a cheaper option

Not just a crap amp, but more importantly a crap speaker and terrible enclosure!
Contact "Fast,not Furious" on here as I just removed his tupperware Bose sub from his beautifully stunning S3 this morning and upgraded it for one of my stealth subs.
You won't find a replacement speaker suitable other than the original Bose unit. It is only 6 inch and needs to be dual voice coil (2ohm per VC) in order to "work" with the Bose amp. The main limitation is the plastic enclosure Audi have made Bose use, it really is cheap and tacky and results in the hollow bass you get from the system. The only real alternative is to upgrade both the enclosure and the amp as per my stealth solution, which dramatically transforms the bass and also the overall sound:
http://www.audi-sport.net/vb/showthread.php?t=78141