can i turn full range speakers in to a pair of sub

i am using a pair of cv re-30 for my rear channels. my hope was to boost effects. i have given up on running speakers as large so the vegas are not being used to their potential. my sub is klipsch ksw12 and is just not doing the job. my receiver is onkyo tx sr 702 mains are nht 2.5i.

here is my idea. i have a harmon kardon avr 20II which can be used as a power amp. i would split the sub signal into 3 and send 2 lines to the hk main in. one to the ksw12. i would run sp wire from hk directly to the woofer in the re30 and use the mid and high section for the rear ch. would this constitute a sub-woofer. 500 Hz and 6kHz crossover points in the cerwin vega re30, does this mean they will not make good rear ch cutoff below 500 Hz. would they make a decent sub with only about 70wpc. thanks in advance.