I've done some more reading on the subject of damping factor for headphones and it was pretty interesting.
The infamous NwAVGuy wrote
very detailed breakdown explaining derivation of the 1/8 guideline that I previously cited. A central assumption of the guideline is that the speaker in question exhibits frequency dependent variations in impedance (in other words; the headphone is a reactive load and has an impedance curve). If a particular set of headphones has highly consistent impedance across the audio frequency spectrum (behaves more like a purely resistive load) then a much smaller damping factor will be required to maintain the intended frequency response behavior. This might explain why some headphones still sound good even when driven by amplifiers with an output impedance similar to the input impedance of the headphone (damping factor of 1).
There are also some who contest the validity of the applying damping factor analysis to headphone systems (though he does concede that the damping factor does need to be taken into account for headphones that exhibit reactance). Here's a quote that I found on the
Head-Fi forum:
"Is an amplifier's damping factor important to headphone performance?
With loudspeakers, the lower the amplifier's output impedance, the higher the damping factor into the rated load. Damping factor is given as the ratio of loudspeaker impedance to the amplifier's output impedance. As the theory goes, the higher the damping factor, the better the amplifier's ability to control a loudspeaker's low frequency response (when the motional reactance of the system is at maximum), because the low output impedance of the amplifier allows any back-emf generated by the loudspeaker to be absorbed by the amplifier. That theory has been discharged by members of the audio community as unsubstantiated.
However, even if the theory were correct for loudspeakers, its applicability to headphones is suspect. John Woodgate, a contributor to The Loudspeaker and Headphone Handbook (1988), had the following to say about the effect of damping factor on headphone performance:
Headphone transducers are resistance-controlled, not mass-controlled like loudspeaker drivers above the main resonance. In any case 'damping factor' is largely nonsense - most of the resistance in the circuit is the voice-coil resistance and reducing the amplifier source impedance to infinitesimal proportions has an exactly corresponding effect on damping - infinitesimal.
However, the source impedance affects the *frequency response* of a loudspeaker because the motional impedance varies with frequency, and thus so does the voltage drop across the source impedance. This means that the source impedance (including the cable) should be less than about one-twentieth (not one two-hundredth or less!) of the rated impedance of the loudspeaker, to give a *worst-possible change* in frequency response from true voltage-drive of 0.5 dB.
The motional impedance of headphone transducers varies very little (or should vary very little - someone can always do it wrong!) with frequency, so the source impedance can be high with no ill effect.
The IEC 61938 international standard specifies that headphones should be driven by a 120 ohm source - regardless of the impedance of the headphones themselves. If the headphones were designed to this standard, then an amplifier's high output impedance should have little effect on the sound of the headphones. In general, headphones with a flat impedance curve over the audio range will not be affected by high output impedance. For example, in May 1995, Stereo Review published a review of the Grado SR125 headphones. The impedance curve of the SR125s, which have a nominal impedance of 32 ohms, varied from 31 to 36 ohms over the entire 20Hz to 20kHz spectrum. Not all headphones may be as well behaved as the Grados, but neither do they usually have the roller-coaster impedance runs of a loudspeaker. Tube amplifiers (with their high output impedances), it should be noted, have very low damping factors."
My TL;DR summary is that
some headphones do not need a large damping factor though it is never a problem to have a high damping factor.