You should use the receivers crossover. That way you can place a high pass filter on your speakers and allow for better integration as well as more clean headroom/wider dynamic range from both your amp and speakers. So I would set the crossover switch on the STF-2 to out. In some instances, you may want to run the crossover on the sub along with the receiver, but for the most part, the receivers crossover should do just fine. In terms of setting, 80 Hz is the typical setting however, it depends on what speakers you're using. If you're using a smaller satellite with 2-4" driver or something like that, you may want to consider a higher crossover setting. Generally, when a sub bottoms out, it's due to the gain set to high, not so much the crossover. However, sometimes if you set the crossover low, say 40 Hz, and you feel that you need more mid-bass punch and turn up the sub to get that, it could cause the sub to bottom out earlier than it should if you had set the crossover higher to bring in more of the mid-bass instead of turning the sub up.