Headphones are not perfectly resistive devices, hence the use of the word "impedance" rather than "resistance". Adding a series resistor changes the frequency response a lot, especially in the bass and lower mid frequencies.
Having that said, I think Beyer headphones sound a lot better from 120-ohms jacks than 0-ohms jacks, regardless of the headphones own' impedance. I guess it's partly because they were designed that way. Thus, the graphs found on Headroom don't reflect the "true" chracteristics of the Beyers, which are what the designers came up with by using 120-ohms jacks.
Why do you say it would be very worse to use a 60 ohms output with a set of 32 ohms cans ? Many argue that damping factor is an issue with speakers because of the much greater diaphargm mass involved, but not with headphonmes. Even then, some of my university professors strongly believed that you should always match the output impedance of your amp to the nominal impedance of your drivers (headphones or speakers) in order to balance the forward and reverse excursion damping factors to give the greatest fidelity. Thinking of it, it makes a lot of sense. People are too used to "dry" bass these days.
A zero ohms jack isn't always the best choice. Also, think about the IEC standard that specifies a 120 ohms output jack. I believe ALL headphone manufacturers should stick with this standard, but as standardisation is optional nobody can oblige them to do so. The adherence to this standard is one of the many things that I like about Beyer, my now favourite headphone company. I wish other companies would say more about how they design their headphones.
OFF TOPIC : BTW, are the Denon D5000/D2000 Made in China or Japan ?
Originally Posted by Cool_Torpedo
A 0 Ohm impedance output jack just means that the device they used to feed the cans with the signal has a very low source impedance. This shouldn't interact with the cans own impedance. It's one of the best possible conditions. It'd be very worse using a 60 Ohm output impedance to drive 32 Ohm cans.