Headphone volume gets reduced

On occasion, I find that the volume on my USB headphones gets reduced... I can not get them to go to the real max volume.  

A number of things can cause this, but one fix that I've used is the following:

• Go to the Sounds interface
• Select the sound device in question and click Properties 
• On the Levels tab, set level to 100%
• On the Enhancements tab, enable Loudness Equalization
• Click Apply

That sound increase (at least in some cases) the max volume of your device.