That depends on what the receiving unit has for isolation etc. my guess is the Aurender has sufficient isolation to not require an add on, I don't know anything about the CD player, and not sure why you need a USB input for it unless your using the internal DAC.
That said Schiit no longer makes the Eitr or at least its not on their web site that i can see. Personally i would not worry about it. Most newer digital equipment has isolation for the usb input now. But if you if can try one then maybe.