Hi Guys, I have the standard Bose sound system in the A4 quattro Saloon, the speakers on the rear shelf sound as though they have seen better days and don't like any music with a bit a base. How do you take the covers off these rear shelf speakers to inspect them. And if shot where and what are good replacements/upgrades to fit the standard holes. (the rest of the speakers are all fine its just the rear shelf ones that don't sound good.)