dal sito GIBSON:

It has come to our attention that a small percentage of “Les Paul Ultra-III” and “Ultra-339” guitars have entered the market equipped with pickups that were incorrectly produced causing them to be “out of phase.” This issue is observable with the toggle switch in the middle position. With both pickups selected (middle toggle position) the output will be weak with a mid range emphasis. In the other toggle positions the output will be normal. Additionally, sometimes other related symptoms can occur as well that resemble improper electronic grounding"

praticamente il problema si riscontra con lo switch dei pickup in posizione centrale, il suono esce piu' debole del normale per un problema di controfase e di messa a terra

qui la notizia completa:

http://forum.gibson.com/index.php?/topic/82728-important-information-for-epiphone-les-paul-ultra-iii-and-ultra-339-owners/