This is how I calculated.
First I plug in a stereo "Y" adapter to make it easy to measure. I plugged in a stereo plug that is open to I can measure AC RMS voltage.
The Machine was set to a constant tone.
With no headphones, measuring the ac output you are measuring without load so you are effectively measuring before the series resistor in the machine. No current flow so no voltage drop).
Next I plug in phones with a known impedance on the other side of the Y cable and measure again.
From these measurements you can calculate the series resistor on the machine.
Now remember, if they are recommending 32 ohm headphones, one reason is because they are available for home stereo's. But also keep in mind that since the Minelabs use tip and ring without sleeve, when you connect 32 ohm stereo phones, you end up in series and you end up with 64 ohms.
The Koss UR-30 actually have about 100 ohms per side. When connected to a Minelab, you end up with 200 ohm phones!
Many 150 ohm phones are in parallel and the speakers may be 150, but the phones are actually 75 ohms, it depends on what brand you use and how they explain their specs.
Then again, there is the speaker efficiency. How loud they are may not only depend on the impedance.