Depending on the crossover settings of your AVR and/or the gain settings on your sub(do passive subs have gain?).
Let's say you set the X-over on your AVR to 100KZ, everything below goes to the sub, everything above goes to your main speakers.
So, therefore your mains will no longer reproduce the low end bass. However, depending on how low your mains go, they may not be able to go as low as your sub, so you may gain some low end.
Passive subs, tend not to go as low as powered subs.
So, you need to find out what the response range is for both your mains, and your sub. If the sub goes lower, then use it, if not, then just use your mains until you get a sub that can go lower.