Ed at Theta technical support is very nice and very knowledgeable. 818-597-9195 is their number. Dunno if they're open weekends, kinda doubt it.
Of course, I'm assuming you've tried it with rca or bnc or some such and verified that both units work. Also, if you have other balanced components (e.g. preamp and amp) you can verify that the cable works. If this were me (and I assure you I'm not suggesting anyone else would do this), I would make sure I hadn't connected the cable to the DAC's output instead of its input by mistake. I do things like that sometimes...
Of course, I'm assuming you've tried it with rca or bnc or some such and verified that both units work. Also, if you have other balanced components (e.g. preamp and amp) you can verify that the cable works. If this were me (and I assure you I'm not suggesting anyone else would do this), I would make sure I hadn't connected the cable to the DAC's output instead of its input by mistake. I do things like that sometimes...