Think about it... your vintage Marshall, Fender, Gibson, Vox, Orange or Hiwatt amp that you cherish for it's tone doesn't sound nearly as good as it could. These amps were designed in the 50's, 60's and 70's when the average standard voltage coming out of the wall was 110v. The whole circuit (tube bias, resistors, capacitors etc) was based on this fact. Today, you could be seeing 125v coming out of the wall socket! This device allows you to 'brown down' the voltage until you reach what the amp should have been seeing the whole time.
Amazing how much it opens up the sound, and allows the amp to operate not stressed out to the max by dealing with 120+ voltage which it was never designed to see.
This is the tone secret you have been missing.