The only alteration I have made to the circuit was to replace a 100k gain control resistor with a 500k potentiometer. With that potentiometer set to its lowest value, hence providing low gain, I went to measure the amplifier output current over a range of frequencies.
when I reached 100 kHz, the output (Green trace in the picture) started to distort. At 60 kHz, the output started to decrease slightly, so it would be expected that the output would continue to decrease with increases in frequency. However, at 100 kHz, it distorts and begins to rise again.