The receiver subwoofer output is variable. I usually do the speaker set-up on my receiver (big, small, distance, etc.) and set the LFE level to zero. Then I put on a familiar disc with plenty of low frequency information and adjust the volume on the sub until I can barely hear the sub over the mains; then back it off a bit (overpowering subs sound silly). This works best with a music CD or DVD.
The other thing to watch for is cascading crossovers between the sub and the receiver. Since the receiver has a built-in low pass filter that engages around 80Hz if mains are set to small, I usually turn the subwoofer's low pass filter up to the max setting and let the receiver do the filtering. If all speakers are set to large, then the low pass filter on the sub should be set somewhere around the roll-off point of the mains (around 40Hz on most subs I think).
Sorry for all of the extraneous information.
I have been reading specs for 5.1 A/V Receivers and I just can't find the answer to the following question: is the does the output level of the subwoofer output vary with the receiver's volume settings or is it constant, regardless of the volume?
Anybody knows?

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