The LFE channel is meant to be 10DB higher than any of the other channels. This is usually accomplished by the Dolby digital and DTS decoders in the reciever, although some recievers have an adjustment called the LFE Pad, which allows the LFE level to be adjusted. This is NOT the same as the subwoofer volume control. In short, the LFE channel should be 10DB higher, but you must also calibrate the subwoofer volume.
Usually having the subwoofer set relative to one of the main channels should be pretty accurate. Here is a link that explains all of this in great detail.
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