I feel that the standard stereo lacks base, i don't want to change the head-unit or speakers though ideally just add a sub. I understand that audi fitted subwoofers to some models. Mine is the standard Audi symphony single slot with integrated tape deck from a 2001 A4(b5). Even if i can upgrade to an aftermarket sub i'd be happy, but i would have thought the standard head-unit would not have pre-outs. Any help would be greatly appreciated.