We're looking at this schematic. The TPA3116 is used as signal generator.

Instead of a 440Ω, we use 2kΩ, we vary the series resistor value.

We are looking for a flat response. The sample with 26Ω does not impress, although it can be argued that most of the yank occurs at frequencies too high for stimming. Results are quite similar between 1Ω and 4Ω, the 4Ω sample perhaps looks slightly better at the high end.

Common wisdom says that if we use a larger series resistor, the box behaves more like a constant current device. Can we measure this?

We choose a 4Ω / 22Ω series resistor and measure the current at varying load.

load	mA (4Ω series)	mA (22Ω series)
0	103.6	25.5
1000Ω	55.6	18.9
2000Ω	38.6	16.4
3000Ω	29.1	12.7

Normalized:

We can see clearly that higher series resistor results in a more constant current device. If there is inconsistent contact with an electrode, a higher series resistor will cause less stinging. But a higher series resistor will also waste tons of power.