I just sold my zbox to another user and upon receipt (shipped via USPS) he advised the device is not working when he installed it in his system.  He installed it between the source (CD) and amp.  He verified the gain was turned up full and he also verified the tube is lit but gets no audio signal.  I also had him confirm the interconnects were configured correctly in/out.  When he removed the device his CD plays fine.

Given the tube appears to be powering up is there anything else that would prevent the audio signal from passing?  Should the audio signal pass if the tube was damaged somehow?

Any advice or input would be appreciated.
Thanks.

I would have him double check the wires again. Looking at the back, the right RCAs are from his CD player. The left RCAs will go to his amp. If that is OK, then have him change the tube to a different one or make sure it is seated all the way. If still nothing than have him take the bottom cover off to see if a wire came loose because of shipping. Those are the things I would check.