A good amp should be fine with either low or high impedance headphones. The special mention however is that amps with a high output impedance (could be any number of reasons) will have particular interactions with cans that don't have flat impedance curves. Typically, this results in a boost to the midbass. Headphones with high impedance or low sensitivity need more voltage swing, and thus may be good at "masking" noise issues in an amp. Higher impedance cans are also less prone to undesirable FR variations with amps that have a high output impedance, this making them feel cleaner or more neutral. That myth is a great example of "correlation is not causation".