1) Generally the source runs out of headroom and can't swing the necessary voltage, especially with portables. It also depends on the sensitivity of the headphone, and listening preferences of the individual. IE, how loud and dynamic does it need to be?
2) Generally circuit harmonic distortion increases with lower impedance loads. Noise floor and hiss can also increase, but I think thats more a factor of circuit gain (so let others dispute this). With tube OTL amps the output coupling capacitor(s) together with the impedance of the output tubes + headphone voice coil form a first order high pass filter that cuts off bass (6db / octave).
To get around this the amp circuit will need:
>A large coupling capacitance reservoir, with a resulting high pass frequency thats lower than the audible spectrum. With good sounding audio caps, at high plate voltages this can be big and expensive.
>High impedance headphone, that will result in a high pass frequency lower than the audible spectrum. This is the generally accepted solution, as OTL amp manufacturers generally don't want to invest the $$$ and volume-space inside for a higher-spec coupling capacitance reservoir. I also speculate that paper-film, high end audio capacitors are not the easiest components to source for moderate-high volume production. So, basing an amp design on difficult to source components could lead to component shortages and longer lead times.
>Output transformer coupling, that will present the tubes and capacitance reservoir with an impedance load that they are more capable of handling. I am not sure why more amp makers don't use OTs. To date theres only two I am aware of under a grand... Mad Ear+ and WA6. Neither amp uses high $$$, "boutique" OTs, although the OTs are still one of the more $$$ components in each design.
But thats all in theory and textbook.... Synergy is a funny, strange thing. Things that aren't supposed to work... somehow end up working. My Earmax for example sounds good pushing loads as low as 65 ohms with the Koss A250 and K701. The added harmonic distortion and lower dampening factor... just sound better with these notoriously bright sounding headphones. I upgraded the lame wall wart with a 24V - 60 watt power supply and it gets plenty loud. The high pass filtering effect just doesn't happen, or at least theres enough even order warmth from the rest of the circuit to offset it. With an HD650 thrown into the mix, its my main late night, low volume rig.