What I suggest you do is get yourself a 4 channel amp and use two channels to run the front speakers and bridge the other 2 to run a sub. Budget on £30 for a cheap amp wiring kit from Halfrauds which will give you power, earth, RCA and remote cables. Only thing you'll need is a pair of RCA splitters (1female to 2male) as the amp will have for inputs and the headunit only has 2 outputs. Depending on the amp you buy you can put a HPF on the fronts and a LPF on the sub without buying any crossover networks