A lot of amplifiers these days, especially in the professional realm, are self-bridged outputs. In other words both sides are driven. You have no choice but to use differential inputs for these. Also some have the negative (black) terminal driven with Inverted polarity, and the red + terminal grounded on every second channel. You really need to suss this out with a voltmeter before connecting any expensive test gear to an amplifier. Remember, a loudspeaker is a floating balanced input. Your measurement should be as close to that as you can get.
I built some test cables and adapter boxes and posted about them here:
Dale Shirk