Hello, I have the FiiO X3ii and the Martin IMS HVA and have tried to use them as DAC's for my Macbook air. But on both I hear a very high pitched humming sound constantly that takes away from the quality and listenability of those two devices. Normally they would both have a quite clear or black background, but when plugged into my laptop using their DAC modes the background has this high pitched hum. I plug them into my mac using a USB cable. I have tried different USB cables, but that hasn't made any difference. I have read that ground loop isolators may help... ... and I have also heard that firewire may help, but I don't know which avenue to pursue. I am hoping to find anyone who has had this USB noise problem while listening to DAC's hooked up to their laptops or computers with USB connections and has successfully eliminated the usb noise and how. Thank you.