Why would the resistor help? It just lowers the impedance and what you'd really want would be a high impedance earpiece. Or is having a useful effect by reducing the damping factor of the circuit?
The high value resistor in parallel with the earpiece provides a working load for the circuit. Without it the audio signal is largely being shunted by the large internal capacitance of the crystal earpiece.
The resistor is not required if using a high impedance moving coil or moving iron earpiece such as that from an old telephone.
Trust me, if using a crystal earpiece the resistor makes a big difference.