I apologize for not thinking of this earlier. Answering another post on a similar topic, I was reminded of stuff I didn't think about when I wrote earlier.
If instead of using the mixer in your Passport 250, you got a small, external line-level mixer, and if your Sub-woofer has pass-through ports (like mine does), you could mix your channels on the external mixer, run both lines to the powered sub-woofer, control the crossover frequency from the subwoofer, then connect the "Thru" port or ports from the sub-woofer into the Passport.
The Subwoofer would take care of the crossover splitting of the frequencies. Your Passport would get louder because it's spared the electrical load of playing the low bass notes, and the subwoofer would give you the thump you can't get from your Passport.
This does assume that your powered sub-woofer has a built-in crossover and pass-through ports. Most do, because most powered subwoofers are used in combination with powered speakers. Your powered mixer, built-in to your Passport, combined with its speakers effectively become powered speakers (the power being in the mixer, not the speakers).