Amplifier speaker outputs: is the black always the chassis?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Two common amplifier arrangements.


  Can I safely assume this or are there any common or likely amplifier designs in which the black terminals might not all be at the same potential?


Bridge mode amplifiers use two amplifiers - one an inverted version of the other. The advantage is that on the same supply voltage you can now get double the peak to peak voltage. This is common in car audio systems where the supply voltage is fixed at 12 V nominal, but it is increasingly common in domestic amplifiers also.

The result is that you can't assume that black is common ground. Investigation is required.