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
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