Dalancroft

Here:

http://www.ebay.com/itm/2-x-Bose-PS18-28-38-48-Subwoofer-Speaker-Drivers-Part-No-279424-Tested-Works-/131269954963?pt=Speakers_Subwoofers&hash=item1e904c8d93
Or if you only want to buy one, here:

http://www.ebay.com/itm/6-Subwoofer-Driver-from-Bose-Acoustimass-5-Speaker-141393-/201154818192?pt=US_Speaker_Parts_Components&hash=item2ed5c2f090
Or here:

http://www.ebay.com/itm/Bose-Powered-Acoustimass-9-Speaker-Driver-6-inch-Powerfull-/301290385461?pt=US_Speaker_Parts_Components&hash=item46264e7035
Or here:

http://www.ebay.com/itm/6-OEM-Speaker-Woofer-Driver-For-Bose-Acoustimass-25-Series-II-Powered-Subwoofer-/161415506006?pt=US_Speaker_Parts_Components&hash=item25951d2856
Or here:

http://www.ebay.com/itm/Bose-Acoustimass-10-Bass-Sub-woofer-6-Speaker-Driver-111791-k-/171269520899?pt=Vintage_Electronics_R2&hash=item27e0757e03
That wasn't so very hard, was it???

Search conducted 9/12, 7 p.m. PST. EZ peasy lemon squeezy! Best,

-Dalancroft

USAVAGEU

Yeah...I've adjusted it every way possible but there's just not enough bass. Ever been to a club or heard live music? It's the bass that you feel in your chest that makes the music come alive
The B&O does a great job with the rest of the sound spectrum but that bottom is definitely lacking.

ex-quattro PETE

Well, yeah, that 6" woofer isn't going to deliver any chest thumping bass.

spijun

Maybe something like this

USAVAGEU

Got THAT right, Pete!

Dalancroft

But better than the paper-thin midrange-style "woofer" that's in there now!

SQive

Well, they can, but not in this limited space.
Give this a try. It won't exactly fit, but you can try it out and see how it sounds in that little box.

Mount two 4-ohm drivers face-to-face, making sure that their surrounds do not touch each other.
(They don't need any extra clearance as they will be synchronized, but they shouldn't touch each other. Also, they should be sealed in between.)
Wire them in parallel, but reverse the polarity on the second driver.
This will produce a 2-ohm setup with the air-moving potential of a 12-inch driver. However, I don't know if the tiny cabinet can handle it.
As I mentioned above, you won't be able to close the lid, and the back of the outer driver will be visible, but it's just a test.
So, anybody got a couple of drivers handy so they give it a try?

ex-quattro PETE

Well, yes, if you add multiple 6" drivers, then that will obviously be better as it's the total cone area and total volume of air pushed that matters. But my comment was about a single 6" driver - there is only so much it can do in delivering really low bass at any sort of meaningful SPL.

By the way, total cone area of two 6" drivers is about 56 square inches. Total cone area of a single 12" driver is 113 square inches. So no, two 6" drivers can't move as much air as a single 12" driver, but two 6" drivers are definitely better than a single 6" driver.

Back when I had my old A4, I bought something called Infinity Basslink for the trunk. It's a 10" woofer with a 200W amp. Nothing fancy, but relatively small package. I'm sure there are better solutions out there these days. The challenge is to find a mounting spot for it where it won't obstruct your access to anything.

spijun

This is the best solution that I have seen to upgrade subwoofer for Q5:

https://www.audiworld.com/forums/q5-.../#post24432437

ex-quattro PETE

I found this shallow depth Kicker 8" sub with enclosure that would likely fit in the area in front of the spare tire:


They come in a 2 Ohm version to match the stock amp impedance; however, I highly doubt the stock amp is powerful enough to drive this thing, so it would require installing a dedicated amp anyway...