Duong, there's no specific best setting for the level control on the sub. Once you've determined either by ear or by meter how loud you want the sub(careful; most setups I've heard had the sub too loud. It shouldn't draw attention to itself)that level can be set by a combination of the settings on the receiver sub output and the control on the sub itself. As Ravi suggested, you should be sure to set the receiver output high enough to reliably "wake up" the sub. If that's no problem, the two controls should be set to a similar amount above their minimum setting.
