My way is simplest becouse i use my old subwoofer box and amp.
So how to make subwoofer?
Simple ! I will show in 3 steps . Step 1. Case the volume controler in box ( i have it becouse it came with sub work's like amp brains) Step 2. Prepare amp just screw it to box. Step 3. Soder speaker and screaw it in place just so simple!
Enjoy and good luck!
Songs = Avici levels and Bass test - Feel The BASS