It is not possible to connect the subwoofers and the front speakers to the same outputs. You will need another small amplifier to drive them. Unfortunately most cheap amps will produce lower quality sound than the stock headunit - which is rated at a very low RMS wattage but has more stringent response and a highs signal to noise ratio (clarity and cleanness)

The subs are meant to produce only bass notes and should have a 'crossover' which filters out all the higher notes that arent bass.

The front speakers are meant to produce everything EXCEPT very low bass which makes them and the subwoofers mutually exclusive.