to ED30DSG, I've just been offered one of these VW speaker upgrade kits like yours, (like the fleabay one), brand new still in box for £25. Like you my main critisism of my standard DVD MFD2 based system (with the 10 standard speakers) is the lack of bass.
When you fitted the upgraded VW speakers was that improved significantly ? I can't really afford/justify £300+ on a nice set of Hertz, Rainbow or Focal components (even though I'm sure they will sound loads better).
The bass has improved slightly - there is a bit more depth there, but the door linings could do with dynomating (or similar stuff) - the woofers are pop-riveted in so if you are going to do mat the inside of the door, buy it before you fit the woofers.
The main improvements are in the top-end clarity and front staging - the speakers must have a higher sensitivity as the front now sounds much louder than it ever was at the same volume with the same track.
For £25 you can't go wrong unless you're after perfection - if so, I wouldn't start with a car as a base :-)
