I’m planning to use the S2 a bit more so I thought I’d upgrade the grotty speakers in my S2 (which are Blaupunkt). My local audio shop recommended some “Kickers Red”. In my student days kickers where shoes but these are definitely speakers. The fit straight in to the existing holes in the dash and rear panel. I did the driver’s side first expected to marvel at my new sound system……. can hardly hear any difference, a bit more clarity and that’s it. Maybe my Apline head unit is grotty but it’s new with BT and connects to my iPhone. I was not expecting a lot of bass as that needs a big driver and I am not after a boom box, I just need to listen to Radio 4 and Radio 6 Music. Amazingly, DAB works fine in my car.
I think it is reasonable to say, the design of the car is not conducive to providing good sound and especially bass as there is no real enclosure in the areas where the speakers are fitted, before I stripped mine for track work, I looked at putting a small alpine sub above the passenger foot rest and another on the parcel shelf to give the car some bass, but I just could not be bothered in the end and drown out the radio with super charger whine 
Something like this