I'd like to add that in addition to the importance of room acoustics/speaker placement discussed above, it is also extremely important to avoid "double filtering" the audio signals to your active sub. The results can be very unpredictable if you feed a sub's filtered line-level input from a processor's filtered LFE output. (Nearly ALL receivers/processor's LFE outputs are filtered). This can either cause the dreaded bass "suckout" or "boom" even if the sub is positioned properly in the room. This is especially problematic when you set the active sub's crossover frequency close to the frequency setting in the processor. The interaction of the two filters, which may have very different characteristics (different slope, etc.), in the signal path can wreak havoc with the sound.
Many higher-quality subs offer an unfiltered line-level input for exactly this reason. It's usually labeled something like "LFE-in" or similar. Or there may be a "bypass" toggle switch you can set to bypass the sub's built-in crossover.
If your sub lacks the unfiltered input and you are going to use your processor to set the crossover point for LFE, you should turn the sub's crossover as high as it will go.
My $.02 ;)
Many higher-quality subs offer an unfiltered line-level input for exactly this reason. It's usually labeled something like "LFE-in" or similar. Or there may be a "bypass" toggle switch you can set to bypass the sub's built-in crossover.
If your sub lacks the unfiltered input and you are going to use your processor to set the crossover point for LFE, you should turn the sub's crossover as high as it will go.
My $.02 ;)