Electrical properties of cables

If you have a sensitive voltmeter, check the voltage being delivered through the cable, both with a load and without.

I also assume you've tried a different set of cables for comparison purposes?

I had a cable like that and like Joe Abrams says measured appx 3 db down. It took me a long time to figure out what was happening. It turned out that if I put some kind of wedge underneath the cable (like a plastic case of a regular camera film), it maintained contact. It was very annoying- sometimes in mid session sound used to shift off center and I would be unaware unless there is vocals in track.

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?

Do measure the Benchmark. It's strange tho that the attenuation follows the cable (as you note in yr 1st post) in some instances and the benchmark output in others.
Overall it looks like a bad connection s/where UNLESS there's s/thing wrong in the B's volume control.

I finally measured the output of the Benchmark and found at full gain it puts out approx. 21 Vrms! This makes sence when you consider dBu = 20Log(Vrms/.7746). That's very high (for professional applications driving an amp without a preamp) when you consider most consumer products put out between 2 and 5 Vrms.

There is definately a loose connection or cold solder joint somewhere in the connector end of the vdH cable that can't pass the voltage. Serves me right for buying used on eBay!

I'll be replacing it very soon.