It may work sometimes, but you need to know quite a bit about the output circuitry of the audio equipment.
Is either the audio equipment or scope battery powered and not connected to anything else? Considerations are slightly different for fully floating gear, but that is not usually the case.
Is the scope input fully floating? Usually the BNC shell connects directly to the chassis, or at least to the scope ground, which eventually connects to chassis. Assuming that is the case, then anything connected to the BNC shell will be connected to protective earth.
First, is the audio equipement transformer coupled on the output? If so, that is a floating output, so it will work if you connect pin 3 to the scope ground.
Does the audio equipment have a protective earth connection (3-wire power connection)? If so, then connecting pin 3 to the BNC shell will be connecting that signal to protective earth, which will be connected to circuit reference inside the audio equipment. So "grounding" that signal, but through a long path. Depending on what kind of output circuit the equipment has, that "long path" feature might be important to behavior.
If the audio equipment does not have a protective earth connection because it is double insulated, then if you do not make any other connections other than the scope it is also mostly floating, but you get some power supply currents from parasitic coupling, so you may not be able to trust the levels of any 60Hz noise (or 50Hz, depending on where you live) will be representative of a true balanced input. But of course you need to connect your signal generator, so that adds the complication of how the signal generator BNC shell connection to the signal generator ground and protective earth interact.
Some of this is generalized for other readers who may come across this thread. In your specific equipment you said that all are either transformer coupled (works fine driving unbalanced input) or That 16xx output drivers, so you should be fine.
One thing that I don't I mentioned in post #26 is that cross-coupled output drivers aren't magical, they will in principle attempt to increase the amplitude on pin2 if you ground pin 3, but the devices still are limited by power supply range, so if you are driving the output to a level where pin 2 and pin 3 are anti-symmetric signals close to the maximum that each output amplifier is capable of, and then ground pin 3, when the pin 2 side of the circuit attempts to increase amplitude it will clip. Transformers are magic in that regard, when you ground pin 3 of a transformer output all the signal amplitude will appear at pin 2 with no clipping worry (although then you have to worry about the input stage of the receiving device, since it may not be able to handle the entire maximum output on one input pin).
