I think my problem is more than a cable. I repeated this test by playing a 1kHz tone off a test disc through the Benchmark and into the Bryston. Everything sounded OK! Since I'm using the variable output on the Benchmark I tried increasing the output when suddenly I heard a sharp transient followed by a very bright harmonic, the order of which I'm uncertain - maybe a 3rd. After backing off the output, the 1kHz tone was now shifted slightly to the left channel just as before. Interestingly, this does not occur with the unbalanced outputs.
I'll measure the voltage and see what's going on. The Benchmark is capable off +29 dBu at 0 dBFS. Maybe I overloaded something but why it results in a shift in balance to slightly off center is beyond me.
I guess I need to call Benchmark again?