even if it is stated that pin 3 should be left open, that’s an invitation to residual captive flutter on the open pin, which will make the highs simultaneously darker and more extended.
The flutter of residual capacitance on the open pin will modulate the return line, as it will be read as signal and thus dealt with by the balanced function itself. And thus micro mangle the transient data of the positive pin’s throughput proper... Basically mis-timed and mis-shaped transient information.
Which will be heard, even if it is very very small, as the ear works solely on interpreting transients and their placement in time and grouped as harmonics in each micro level placement in time.
That’s how we hear. We only process about 10% of the audio signal and all we process is the transients and micro transients, in level and timing placement.
So the tiny flutter of an open pin three, is going to produce a signal that is heard. People have various learned and inherent hearing skills so some might not hear it but I think most will.
Maybe not ground it but tie it down through high resistance of some sort. Cut out any possible micro-capacitive flutter, if one can.
Basically, it depends upon how pin 3's open condition is dealt with by the manufacturer. How sensitive it is to potential modulation. That we see many circuits, even if they are resistively loaded (even when unplugged) , they can have caps or plugs put on them, to block stray signals and such from occurring, or interfering with the open condition.
