Passive speakers are naturally balanced, the necessary conditions being (a) stray currents induced in the cable see the same impedance on both wires, therefore

Passive speakers are naturally balanced, the necessary conditions being (a) stray currents induced in the cable see the same impedance on both wires, therefore producing identical voltage deviations (common-mode noise), and (b) the sound produced by the speaker is a function of the voltage difference between the two wires. If induced noise is a problem in such a setup, the actual improvement to be made would be to use twisted pair or shielded cable, rather than bare parallel wires. This helps eliminate differential-mode noise due to near-field EMI which parallel wires are susceptible to.