I plan to get an Mp3 HU ---> Pioneer DEh-4750
then a sub Kicker S8L7
then an amp to power the sub either kicker or Lighning Bolt.

Questions:
1) with the current setup, what seps are good?
Probably SPL muna kaso with the 8 incher sub, baka SQ+ na. I actually don't know the outcome of the setup after this but I'm willing to experiment.
2)is it still advisable to get a separate active crossover vs. the crossover of the HU? And, what are the pros and cons?

Vehicle is a gen2 '01 strada currently with JBL seps up front and OEM Mitsu speakers at the rear.