Just wanted to remind folks about a problem I encountered with AKG headphones years ago. The entire AKG headphone line is out of "absolute phase" with the rest of the world. This can be easily verified by placing one headphone cup over your mic and then sending a 1khz test tone from your daw to it and then record it. Assuming your entire chain has been checked for phase this will result in an out of phase signal being recorded back. This is a problem that only the singer will hear while in record, there is no apparent change in the control room. The problem occurs on your eardrum and is volume dependant. Another way to test is to put the headphones on, record a phrase and then immediately play it back thru the phones. If it sounds differen't then flip the input phase and try it again. The out of phase soundwaves cause cancellations on the eardrum and can make the cans sound very weird. I emailed AKG in Germany years ago and they confirmed my findings and said that it is still done for "historical reasons". I fixed mine by opening the cans and reversing the +- leads in both sides. It can also be worked around by flipping the phase somewhere in the chain before it gets monitored by the singer.