Measure AC power consumed by a loudspeaker
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Test setup avoiding earth-shorts.

Figure 1 shows one way of making the measurement and avoiding the grounding problems. It does suffer from the disadvantages of modifying the circuit under test as you pointed out in your question.

Note that even this setup could have problems if there are high currents in the return wire as this will raise the voltage and some of the current may return through the scope ground and the mains earth wiring.


  Ground reference for the oscilloscope must be on the star ground point of the amplifier to avoid the oscilloscope reads the voltage drop on the speaker return wire. (As commented by soosai steven.)