I want to connect my digital piano to a Mic IN input using the AUX OUT outputs. Reading through the keyboard's manual I found a reference that states one should use "zero resistance plugs and cables" when using the AUX OUT outputs.

The opposition that a substance offers to the flow of electric current. All electric systems display a certain amount of resistance. The standard unit of resistance is the Ohm.

Pro Line microphone cables CPM # FM - technical data
This cable is equipped with these
NEUTRIK connectors. Cable used: CORDIAL CMK 222
Conductors: 2x28x0,10mm = 0,22 mm2
Shield: Spiral Wound Copper
Overall Diameter: 6,4 ± 0,2mm
Capacitance Cond./Cond.: approx. 70 pF/m
Cond. Resistance: < 85 Ohm/km Ohm/km