When I purchased my Ultrasone headphones..I listened to a demo CD...Without exception..everyone that listened to the demo and with the Ultrasone HP's...turned around to look where the sound behind them and to the right was coming from ( clarinet )...The stereo field when done right is totally amazing...
Now applying this to a live set up could be another story..but still way better to have stereo over mono..or two mono signals to two speakers..that you think is stereo..