Sealing your fullrange speaker into a smaller container would reduce the amount of distortion. Now its time to solder the 47uF (25V) capacitor to your speakers as a simple crossover (frequency limiter). Solder the black wire to the negative terminal of your speaker, then solder the positive terminal of your speaker to the negative side of your capacitor, after that solder the red wire to the positive side of the capacitor. Just follow the simple schematic diagram below.
I have a second version of my boombox. There were lots of improvements and revisions, including the crossovers and speakers. The crossover is now a "2nd Order Butterworth". Every part of the 2nd version is top secret, its my secret recipe. This version in this instructable is just a prototype, so please do not complain why did I just add a very small capacitor for my crossover.
Summary: Add a separate enclosure for your fullrange speaker, this would reduce distortion. Also add a capacitor as a simple crossover.