In a scenario if these speakers are not being used through the onboard amp, what would the cross over be set at? Would the passive radiators work or would it just be a larger book shelf speaker?
I am pretty sure those are not active speakers. Sorry, I'm saying that wrong. Most Def-Tech speakers are a passive speaker with a active subwoofer integrated into them for the models which have an integrated amp. I'm not fully knowledgeable in all of their speakers, but I believe that's the case. Typically you run both line level and speaker level to their speakers. So, their standard speakers inside the enclosure and run in a traditional manner. You can run them off any AV receiver just fine. They often sound good, and sometimes sound excellent. My BP30 speakers were very well reviewed and considered one of the better values for home theater at the time of purchase.
While you lose the low end base 'full range' feature, that's something you can fill in with proper subs in the room. I'm not completely sure how my final theater will send up working, but at this point I have several 10" subs along with a 18" model which I will be trying to sort through for best response.
But, please let us know specifically what model you have and maybe a link to the product page/owner's manual so that we can give you some more specific feedback on what your options may be for best overall performance.