Replacing factory subwoofer for '02 A4 Avant

Hi there, I have an '02 A4 Avant with the Audi Symphony stereo. I would like to upgrade the speakers using the current mounts available, i.e. I don't want to place the sub in a separate box. Has anyone done
this? What size sub can fit in the current location? Any suggestions would be much appreciated. Thanks! Jeremy

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.