I've done exactly what you don't want to do Oli....changed the head unit for double din Kenwood nav 7200 and drive a small 10" sub in the boot via a small Rockford Fosgate amp (which is easily removable as I have 2 kids). The standard interior speakers remain oem. I've always put decent sound systems into my cars and replaced speakers but I have been amazed how this set up sounds. Perhaps it could sound even better with upgraded interior speakers but it's now more than acceptable - even without the sub, bass is quite strong and clarity good
I fear that you could replace the speakers but keep crap sound. As I understand it, the oem head units 'clip' the output which means as the volume is increased, the bass is reduced to avoid blowing speakers.
Rainbow make some good oem replacement speakers at reasonable cost - straight forward swap type. I have heard of these recommended by a couple of other people with Golfs.