My stereo is the 6 speaker set up at a guess. 2013 28i. Proffesional stereo.Playing usb is far too quiet on most tracks. Sound is not the best either. What is my best option for a decent sounding stereo. A stealth sub perhaps. ? Could an amp with speaker updrade be the answer? Would I be able to keep head unit? What sort of costs are we talking to updrade to bmw hifi system and is it worth it.