I recently purchased a 2012 A3 and I am less than pleased with the sound quality of the Audi Concert system. I am looking for suggestions for upgrades but would like to avoid replacing the entire system if possible. This will not be a DIY project. Thanks!

 

The easiest single upgrade is to add a proper subwoofer, or a stealth sub. This not only provides much punchier bass but frees up the underpowered cabin speakers so they can run with less distortion at higher volumes. Personally I would add a stealth subwoofer install a decent 4 channel amp and upgrade the front components JL/Rainbow/Hertz/DLS keeping the stock HU. The 2 big problems in the non Bose system is lack of power to the speakers (2 x 20w peak) and the feeble plastic tupperware sub in the boot. The HU is actually fairly decent as a source, everything else is pretty poor. The speakers are mounted to the inner door skins, so with solid MDF adapters and half decent 6.5" components you can get very good mid bass from them once amped up.
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Audi A3/S3 8P MDF front speaker adapters 6.5" component | eBay
Cheers, Andy