The short answer is the headphone Z needs to be equal to the diode Z plus the generator Z. You have little control over the headphone Z. (I have a high Z and a low Z headphone and high Z and low Z earbud.) Chances are the circuit will need to be adjusted to the phones. The diode Z can vary with diodes but here again choices are limited. Your main control in the situation is the coil taps. Tapping up or down to adjust the generator Z will give results.
* After making this post I looked at the datasheet and found the Rds is 104KOhms. The the original poster told me the selectivity is very poor with the diodes. This tells me I went at it completely backwards. The diodes are loading the tank dropping its Q and spreading it bandwidth. The MOSFET reduces the loading and allows the tank to boost the signal. In the final analysis the tank load needs to be light to get the Q needed to boost the signal and this allows the headphone to draw more power without killing the signal. It seems a bit contradictory light load means more output. 100% of zero is zero but 10% of 1 micro watt is 100 pico watt. The old headsets worked quite well with less than 1 micro watt.