From what I remember the loudness level desired and headphone sensitivity set the output power need to reach said volume. This has nothing to do with resistance.

From the power requirement we work backwards to find the voltage needed. This is seen going from the Joule's law ( P = V^2/R for constant voltage sources like amps ). From there we can start to see that as resistance goes up the power draw from the amp goes down. Too high of resistance the too much power draw will be needed to reach a specific volume.

Working with Ohm's law and understanding the voltage needed from the Joule's law calculation we see from:

V = I*R that as the resistance goes up the current will go down to keep the voltage constant. This is one reason high impedance cans can be a good thing so they do not draw too much current from a source. This was a design implementation in years past to allow multiple high impedance headphones to be plugged into the same source without drawing too much current from that source.

In the end it depends upon whether the current is the limit or voltage. Take a portable player. It may be limited to just 5 volts peak to peak out. This means that if for some reason more than 5 volts is required to get to a specific loudness then there is no way the source will provide it. This is a crude example, but you get the idea.

You can read more here:

http://en.wikipedia.org/wiki/Headphones

and

http://www.apexhifi.com/specs.html

In short you go from the SPL using Sensitivity to find the power requirement, from there the current / voltage can be acquired. If the amp can deliver both the current and voltage required to achieve the SPL desired using the impedance of the headphone then it really does not matter what the impedance is of the headphone.