With so many cars now having custom-fit built-in audio systems, it’s sometimes hard to know how to upgrade them – but Alpine has the answer with the PWD-X5. Small enough to be hidden away under a front seat, it combines a digital signal processor, an active subwoofer and four channels of amplification, and can be fed with either speaker-level or preamp-level connections. It even comes complete with the all the cables required. Adjusted via the supplied wired controller or a smartphone app, it also has a dongle to add Bluetooth to your In-Car system. This is the easiest, and most effective upgrade you can make.