I first read this and thought that what JC suggested was the same as what was done. But I think the difference is in the receivers sub trim level, vs. the LFE level.
If the receiver has an LFE level adjustment, it would not be applicable while you are listening to music. The LFE is a separate channel (the .1) in 5.1 multichannel audio/movies. Watching a movie, this adjustment would apply- but not when listening to stereo CDs.
Look for the trim or level adjustment for the sub in your receiver. It should be in an area where you can adjust the level for all of the main channels. Turn the sub level up in the receiver to send more source signal to the sub to keep it awake. You can then turn the gain on the sub itself down to balance it.
