No sound from Pro-Ject Pre Box RS Digital Toslink Inputs

Hi audiophiles,

I have a Pro-Ject PreBox RS Digital, and I am unable to get sound from any of the Toslink input ports. I have connected the Toslink cable to the optical out on my TV and run it to the optical input on my Yammie Aventage receiver with success. Connecting the same cable to any of the Toslink inputs on the Pro-Ject yields dead air. What's really strange is that the DAC indicators light up when I plug the Toslink into the Pro-Ject, so there is a connection being made. I have walked through all permutations of sound output setting on the TV, and then walked through them again with an audio installation technician on the phone: nothing. I guess the next step is the repair shop, but I thought perhaps someone has had a similar experience, or maybe there's something I'm doing wrong...this is the first time I have tried to connect via Toslink. I bought the Pro-ject used from an Audiogon member, so I am inclined to believe that I was sold a working unit...