instead of setting the axiom ep500 to bypass, try setting it to 80hz, i noticed a huge improvement in lower bass by doing this, not sure why,i believe it has something to do with regards to a cross filter in the outlaw 990

Interesting. What are your crossover settings on the 990?

990 is set to 80hz
however it is also set to lef + main
all speakers are set to small

If your mains are set to small, your sub should simply be set to "on". If your mains are large, then the LFE+main setting will definitely yield a reinforced bass signal - in some cases (as with a big sub like the EP500) that may be excessively reinforced. In that case, adding a second crossover at the same point could tame that reinforced bass and yield cleaner sound. It makes sense - not a typical approach, but that doesn't mean it's a bad thing.