I have a Darkvoice 337 with dual volume knobs (dual monaural), but it seems that no matter what I can't seem to balance the left and right. It seems that the right side is harder to drive than the left, because one twitch on the left equals 9 to 11 oclock on the right. Is it just me, or the amp, or the headphones? Should I try other headphones? There is a bit of a hum on the left. Could that be throwing me off?