Unfortunately you'll struggle to get any bass from the OEM speaker locations in the front without major custom work to the door panels to give you more depth and something solid to bolt the speaker to.
There's plenty of room in the rear panels and you could actually mount 2 JL 6W0 subs in these locations but the feeble hardboard rear panels would struggle to support the weight, and you'd need another amp to drive them.
I thought about fabricating an MDF enclosure behind the rear panels to take a sub but never got round to it.
I found by uprating the front components with semi decent aftermarket units (Infinitys or even Pioneer custom fit which are a doddle to install) and then running some meaty mid woofers in the rear (I had Alpine 6.5" SCS units) all driven by another 2 or 4 channel amp behind the other rear panel, you get a really good punchy sound. But the limitation on the fronts will be the amp/HU if you are still using the Audi HU. It only has 2 x 20w for the front speakers and even with another amp the SQ will be limited. The next stage is really a decent HU which will have a sub out do you can control the sub independently and more power onboard for the cabin speakers. There's no point spending a lot of money on the speakers if you're driving the fronts off the puny Concert.
