Sound system upgrade

[Maffikus]

Hi all. OK I got my XF around a month back and I want to upgrade the speakers in there. Even though its a front amplified system the sound in there is real weak. Its quite a tinny sound concidering its a half amplified system with no specific speaker brands on the grills (super bog standard I'm guessing) and no spare wheel sub. 

Firstly, I followed a video online to add a sub to the system using a LOC and tapping into the speaker wires at the amp. I connected to the rear speakers as they did on the video but the sound output on it was pointless, hardly any difference. I then took the taps off the rears and attached them to the fronts instead and had an awesome bass increase what sounded great. Only snag then was jumping in the drivers seat and having the dash throw a "park brake fault" warning at me. I disconnected the sub after that. Next day I drove to work but the error still showed up so it went to the garage. They said it showed no errors except for a slight voltage drop what was wierd but came back and been fine since. They said that a tiny drop on battery voltage can make the system have a wobbly and throw errors at you but it' wwierd that it gave a brake error not stereo error?? 

I'm kinda giving up on touching electrics after this as they are so sensitive these days on newer cars. For a speaker upgrade, would anyone suggest going to an audio specialist or would I be safer going direct to jag dealership and get some upgrades knowing how sensitive the system is? It's really annoying seeing other jags doing this really simply with zero problems but I end up being the unlucky draw. Also if I picked up a genuine wheel space sub, is this just plug and play or would it need certain upgrades to allow it to be connected and picked up by the head unit? 

A but long winded but any advice here would be amazing. 

Cheers!