Another method would be to put a 10k resistor in each cable; then if both
sources are powered up, they would see a 20k load and the amp would see
aproximately a 5k source resistance. With only one source powered,
results are unpredictable.


I've done that back in my distant past...

Works fine if both sources are powered up, and one outputting silence,
but if one source is not powered up, the ESD diodes in its output op-amp
will start to conduct at above and below 0.7V (as the supplies are at
zero volts), and that will make for horrid non-linear clipping or sorts.