Penso que não dá para aumentares apenas o Volume do Sub... mas, como tu não tens BOSE, podes sempre "obrigar" o Rádio a tocar no Modo-Bose (com "equalizações" diferentes) para ver se há algum tipo de alteração que vá ao encontro do que pretendes. Precisas é de um Cabo OBD...
Standard/Bose and other configuration modes:
You can program the Concert II radio to use either the standard or the Bose equalization setting, as well as a few other basic configurations. This requires the use of a VAG-1551, VAG-1552 or equivalent scan tool. Knowledge of the usage of such a scan tool is assumed.
A- Connect scan tool to the car's OBD-II diagnotic port.
B- Turn on ignition and radio, select Radio Controller in scan tool.
C-The radio shows "DIAG" in the display.
D- Select "Recode Module". You may enter a 5-digit code according to the table below. For example, "00007" is a standard setting while "00017" is a Bose setting, both with CD changer and secondary display.
E- Turn off radio and ignition, disconnect scan tool.
F- Turn on radio to hear the effects. In Bose mode, the radio display will show "BOSE" briefly after turn-on.
Digit Description:
1 Country ID (0 = standard)
2 Sound Coordination (0 = standard)
3 Number of passive speakers
0 = No passive speakers (Bose)
1 = Front left (Bose with hands-free phone)
2 = Fronts passive, rears active
5 = Fronts passive, rears non-active
4 Sound System
0 = standard
1 = Bose
5 Radio Configuration
1 = No CD changer
3 = With CD changer
5 = With secondary display
7 = With CD changer and secondary display
Note: Programming the radio to run in Bose mode does not make it a Bose system if it wasn't installed at the factory as such. The Bose system also has an different external amplfier and different speakers.
in
http://www.ibiblio.org/tkan/audi/radios/concert2.html
Ou seja, no tal numero de 5 digitos que te aparecerá, o 4º número deverá estar a 0... e terás que o alterar para 1.
8)