I am slowly gathering the pieces together to build my own sub(s).
Sure you can mix different subs together. It probably depends on the room as much as anything how effective that will be and how much EQ you will need.
I wouldn't bother with the EP500. First, you have no idea what the driver or amp specs are so have no way to properly model the system. As Tom pointed out, Axiom has already optimized the sub for this driver amp/dsp and cabinet combination.
Hometheatershack.com, avs.com and diyaudio.com are the three places I would go to learn about building my own sub.
