[Help] Channel imbalance with Fiio e9

I just received my Fiio e9 in the mail and paired it with my e7 to try out with the headphones I have (Sennheiser HD 555 and HM5 clones) but I have noticed a channel imbalance with the right ear-cup being louder then the left on both headphones at both gain settings. Is this a manufacturing problem or is it due to the headphones having a low impedance so it would just be better to use the e7 until I receive my HE-400's.

Does anyone know a way to fix this or is it best just to RMA the amp.

EDIT: I have noticed that I only get this channel imbalance when using the USB line in only not the RCA line in with my phone. Any ideas?