Not to go too deep into details, but I just want to warn people about considering impedance alone as the only factor deciding 'how loud' or 'how easy to drive' the headphone will be. Sensitivity matters as well. For example, AKG K701 is only 62 Ohm rated impedance, but sensitivity is around 90 dB/mW SPL (quite low) which makes them quite power hungry.
Both open and closed backs can be 'head clamps'. Typical discomfort in case of the latter comes from the fact that there's no 'airflow' causing more pressure build up (air pressure when the sound is generated, not the headphone 'pressing' on the head by itself).
Still, I agree that open-backs are better choice unless someone's listening in really loud environement. In such case open-backs would require really high volume levels to 'hide' environmental noise which is not healthy by any mean.