Subs provide speaker level inputs because equipment varies and not everyone has a receiver or pre-amp that can perform bass management.
If you use speaker level inputs, you have no choice but to use the sub's internal xover. You didn't mention the type of receiver you are using, but if you want to use speaker level inputs, you have to tell the receiver that the front speakers are Large and Subwoofer=No. That way the receiver will send a full-range signal to the sub (which is connected to the receiver's front left and right speaker terminals) and the sub's xover will deal with sending the lows to the sub and the highs above the xover to the front speakers.
It will sound the same as when using the line-level input to the sub from the receiver's subwoofer pre-out, but you have the disadvantage of all the extra speaker wire - not to mention that you are sending the sub an already amplified signal which it will step down to line-level, do the xover, then send the frequencies above the xover to the speakers connected to it.
If you use the receiver's bass management, you set all speakers to Small and Subwoofer=Yes. You then set the xover you want in the receiver. The receiver will take care of sending the sub only the frequencies below the xover and it will not amplify the part of the signal that includes those frequencies (the sub does that). That takes a huge burden off of the receiver and in general allows the receiver to play louder and cleaner. The receiver also does time alignment (this is where you tell it the distance to each speaker including the sub). Time alignment helps to balance the hand-off between the frequencies the sub plays and the frequencies the rest of the speakers play.
So, 99% of the time it's better to let the receiver do the bass management so the sub only has to amplify the frequencies it will reproduce and the receiver doesn't have to waste power amplifying frequencies that will not be sent to the other speakers. Because the receiver is handling the xover, you want to disable the sub's internal xover (you said yours has a switch to do so); if you can't disable it, then set the sub's xover as high as it will go to get it out of the way.
Besides the fact that modern surround receivers are designed to work that way, you can make adjustments on the fly with the receiver's remote instead of fiddling with the dials on the sub itself.
No offense taken, just trying to clarify things for you.
