I would definately upgrade your headunit first, then speakers (even £30 ones will be better tho), then fit a sub.
When making your headunit decision, get one that has sub woofer crossover and hi pass filtering on the speakers, it will make a world of difference to the soundstage. Checkout the Pioneer Reference Series.
Then sit down and start tinkering with all the faders and balances to get the soundstage just right. I generally use more front speaker than rear (rears are only for fill) and set the crossover around 100Hz, but it all changes with different cars.
As above, you will need a couple of ISO harnesses, makes it plug and play. :wink:
I wouldn’t worry about DAB, really quite bad quality.