I unfortunately have to use headphones sometimes. I am testing a set up in preparation for my new amp. Basically I have an XLR to 1/4" cable. XLR Out from my amp > 1/4" with a 1/8" adapter into the light blue "Line in" jack on the back of my Soundblaster Audigy Soundcard. I only get sound out of the left channel. What do I need to buy or do to get sound to play through both headphones?