So I set my main speakers (bookshelfs) to small and set the crossover frequency to 100 Hz. Since the receiver is connected to my computer via an SPDIF cable, I decided to test if the crossover was working by downloading a program that generates tones at desired frequencies. Now forgive me if I sound like a newbie (because I am), but when I generated frequencies below 100 Hz, I could still hear it on my main speakers. This isn't supposed to be happening right? I could hear it on my speakers all the way down to 50 Hz or so. Shouldn't the receiver send all info below 100 Hz to the subwoofer?

Thanks for any help!

It is OK. The crossover is not a brick-wall cutoff but, rather, a gradual slope. So, 50Hz, which is only one octave down, will still have half the output or so.

Oh, I did not know that. Thanks for the info! Now it makes sense.