My passenger one has crackled for years
Not just mine then!
There are loads of speaker options available for the MK5. I think the main area of faff is VW rivet the plastic speaker basket to the door, which just usually need drilling out with a 5 or 6mm drill. No drama really. Rivet gun kits are cheap as from Halfords, or you can go full ghetto and silicon the replacement basket to the door or use self tappers
As mentioned above, the head unit itself is the weak link. The amplifiers are very weak sauce and distort easily. That can be solved to some extent by having the fader on full front, therefore the amplifier has less work to do. Rear speakers sound sh1t anyway. No one sits with their back to a concert, so shove all the amplification to the front only and see if that helps