Kores, como te quemas. Lo cierto es que Naiet puede tener razón en algún caso (platos y fonos sobre todo), los bucles de masa son así de caprichosos.
Otra forma ligeramente distinta de controlar la polaridad, esta vez en inglés ....

AC POLARITY MEASUREMENT METHOD

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You may adjust each component to be properly polarized by following the described method.

As much as possible try not to use adpaters for connecting your equipment in reverse polarity but try to have an AC plug rewired properly. Use only adapter to determine the proper polarity first.

POWER AMPLIFIER or ACTIVE SPEAKER POLARITY

First to adjust are the final electronic units of the system. Either the power amplifiers, or the active speaker if the amplifiers are built-in.

Use a DC Voltmeter with high impedance input.

Leave all components connected to the AC line and turned on. Disconnect all analogue interconnect or digital interconnect cables from the preamplifier or processor to the amplifiers.

Measure the residual voltage between the signal ground of the amplifer inputs and the its chassis/earth connection. Select the AC plug position which minimizes this voltage.

PREAMPLIFIER or PROCESSOR POLARITY

Second to adjust is the center unit of the system. Either the preamplifier (analogue or digital), or in a Home-Theater system, the Multi-channel processor.

Leave all components connected to the AC line and turned on and all analogue interconnect or digital interconnect cables disconnected.

Measure the residual voltage between the signal ground of the preamplifier outputs and the signal ground of the power amplifier. Select the AC plug position which minimizes this voltage.

SOURCE DEVICES POLARITY

For each source used in the system, repeat the following adjustment. If a D/A converter is between the DVD or CD turntable and the preamplifier, make the adjustment on the D/A first (Turntable disconnected), then on the tuntable.

Leave all components connected to the AC line and turned on and all analogue interconnect or digital interconnect cables disconnected.

Measure the residual voltage between the signal ground of the preamplifier input and the signal ground of the source component output. Select the AC plug position which minimizes this voltage.