#1
1. A P bass ickup is just a split single coil, correct? So could you reverse one and essentially make a humbucker of sorts? (Im talking about using 2 pickups)

2. This one may be kinda retarded, but I could just use a sub to replace my bass speaker with, no? I know, match the ohms and all that cool shizz.
#2
1. You'd have to reverse the coil widings then the magnet polarity. In short: no.

2. You can use any speaker you like as long as it can handle the wattage. A bass speaker basically IS a sub.