These mixers, both 2600 and the MKII versions need to be placed on a flat surface that will be wide enough to have all the feet under the unit placed on it. If these boards are placed on an uneven surface, or a stand that presses on the frame, it's possible to short out part of the headphone circuit and some of the solo functions as well. I've found this out from our evaluation of the new boards coming in as well as the unit in the Tascam demo room.