If the amp is able to be ran in tri mode he can wire the 6x9's left and right, and bridge the sub with a passive subwoofer crossover safely. I dont know how it would sound though. If the 6x9's can only handle 50-70 rms and the sub can handle 250 it may not even out.
check uot page 53 of this site you can use the bar to the right to go directly to it or read up to it and learn everything there is to know about car audio if you want. thank the member (gsm13) for the link cuz he's the one I owe thanks to for giving it to me.(
www.bcae1.com) Just call him "the link master".