Any ideas what the size of the speakers are in the Superb? I think its time for a change

The weak link in the Superb mate is the headunit, the speakers are more capable than you think! :P

Also worth bearing in mind that they are rivetted in, so would need drilling out. (But you know a man who can )

Would back that up ....changed my head unit for a Blaupunkt and the sound changed totally

I thought they would be rivetted, my old arosa was the same. Soon sorted that though. I'm thinking of installing a decent motorised screen linked to a small pc in the boot running itunes, IE and a TV tuner etc.

Try the headunit first, if you feel bass is lacking get a good quality subwoofer amp and subwoofer (an all in one super box for £89 isn't going to sound great and will be all boom).