Double bass is a preference thing, some like the doubling up of those frequencies, some don't (I prefer not to use it myself). Same pretty much for a crossover setting., it can boil down to preference. I didn't realize you had those powered woofers (hard to call even DefTec's dedicated subs "subs", tho let alone their powered woofers). Experiment with both....
Also, rears are for when you have a 7.1 setup, for 5.1 the next two speakers used would just be surrounds (and are generally placed more to the sides of the listening position).