basically yes, u can use everything stock and add an amp and a sub.

As for the S60, any amp with 50w rms per channel will do. The rest is up to tuning, and budget to get a better amp for better sound.

dun get it wrong, crossover does not produce power, it just deal with frequency going into each channel. A monoblock is an amp, a 1ch amp to be exact. So u can use it for ur sub of course, as for how many watts, that depends on your sub.

for sub, something like kicker cvr or rockford fosgate p1 series can d.
as for the amp, i have no interest among them, not a single one of the kole is written in rms, onli max power, perhaps u can try the JBL CS60.4 amp or a cheaper option, blaupunkt GTA 480. dun waste ur money on cap ayam or itik, u will regret it if u progress further into ice.