Downward firing subwoofer

Probably not.  The passive sub is designed to work within the cabinet volume and the subwoofer driver.   Removing the passive will really muck things up.   If you need more bass, get another sub, preferably the same as the one you currently have.   

Who makes it? Why would you want to? Is the passive in bad order?

Regards

Passive just a unwired driver. 
   Buy a better sub.

@spatialking awesome, thanks for the feed back.