Can someone please clarify the charging mechanism for the 627x? Is it possible to 'disengage' the battery and purely run powered from a USB port on my computer? I am looking to the 627x as a 70% desktop, 30% portable amp. Also, there seem to be two switches on the faceplate. The instructions only mention one gain switch. What does the other one do? Also, any comments on noise floor and balance with sensitive IEMs? Thanks.