Hi everyone. I own a 2013 1.4TFSI A3 and I'd like to upgrade it's sound system because the stock one is too low in quality for me. I currently don't have enough money to invest in a complete B&O speaker package so I'd like to start slowly investing in it. Can I start by purchasing a center speaker or subwoofer & amp? Will it work with my current speakers or do I need the whole set? Any advice would be awesome. Thank you!