Are you talking about this schematic?
In this case, the resistors are in series meaning the amplifier sees a maximum impedance of 33 + 3.3 = 36.3 Ohm.
Since the headphone is parallel with the 3.3 Ohm resistor, you would calculate the total impedance (adapter + headphone connected) as such:
330 + 33 is the same in attenuation as 33 + 3.3. Unless you want to run some low impedance headphone off an OTL tube amp, I don’t see a reason to do this.
330 + 3.3 would give you roughly 1/100th the signal, which may result in barely audible volume levels.
If you are willing to build a slightly bigger device, I would suggest “copping” (=it is a variable voltage divider, no magic in there) the ifi IEMatch.
The 16 Ohm resistors would give you half the output signal, the 3.3 Ohms give you 1/10th
