The answer is in this paragraph from the original article:
The sensitivity of the amplifier measured at 1k Hz is 6.5 mV for an output of 2O W when no feedback is applied, and approximately 22O mV with feedback, the loop gain being 3O dB. The loop-gain characteristic of the complete amplifier for the full frequency range is shown above. The sensitivity, with feedback, at the overload point (27 W) is approximately 26O mV.
So if your source such as a CD player has an output voltage greater that 260mV then you can replace the R1 with a 100k pot. The standard output voltage of a CD player is 2V so adding a preamp will simply add noise and distortion to the signal chain.