Hi Rivieraranch,

The reason the Taboo does this is because output transformer negative posts do not connect to ground meaning they float. In addition there is a feedback loop from the negative post that connects to the cathode of the input stage. If the negative binding post is grounded on either channel it will modify the value of the cathode resistor on the input stage causing it to have more gain which is why you hear it get louder.  

This feedback loop is modified by lucid mode which connects both negative binding posts together through two different values of resistors. This allows feedback from the two channels to bleed to the opposing channel at a controlled rate determined by two lucid mode switches. The result is perfect cross-feed for headphone listening.

Depending on the position of the original lucid mode switch, this bridge between the channels is broken or engaged. If the switch is off, you would only hear one channel get louder instead of both. This is also why when using a standard 1/4 inch headphone jack, lucid mode should be turned on, otherwise the ground for one channel is dropped and the image will pull in the other direction.  Put another way, the headphones will work with lucid mode off but won't sound right. With the balanced headphone jacks, you can use your headphones with lucid mode on OR off.