easy, cheap and better rarely go together!
I have a huge kicker 4x amp and lovely MB Quart components & sub sitting in my
garage - which I have been toying on fitting - but the standard set up sounds quite decent I haven't really needed to (I don't have seat sound).
The Leon has big 8" door speakers, and tweaking the bass up a smidge on the EQ really sounds good to my ears - and I do like nice sounding music, having some nice Ruark home speakers.
I would add, bluetooth music from my phone sounds much better than the DAB radio.