According to John Doran's webpage, the original amp design had a flaw in it. Should you connect the Morgan regen receiver to the two-tube amp as shown in the book, it will fail to work. This is because the headphones are the actual plate load for the tube, in the receiver, which provide a path to the B+ voltage. Since the amp is capacitively coupled to the input, there is no path to the B+ voltage for the plate anymore. A 2 k ohm resistor placed across the amplifier input terminals rectifies this.
I also incorporated his modification to capacitively couple both inputs, to prevent the B+ supply from being short-circuited should the connections between the amp and the receiver be swapped. It is a good thing I did that, as the very first time I hooked it up, I had it backwards (and the audio was actually less than what the receiver itself produced).