LizardKing, to clarify (or confuse) things further, tube amps have a wide variety of output impedances. A good transformer-coupled amp can get output impedance down near zero. On the other hand, one of the cheapo OTLs might have an output impedance of 150 Ohms or more while the manufacturer tells you that it works with "anything." Riiiight. Then you'll have something like the Zana Deux which has an output impedance of (IIRC) 12 Ohms, but it is an OTL. That's because the 6C33C has a naturally low output impedance, so you get the benefits of an OTL while being able to drive low impedance headphones.
To make things a little more confusing, the old standard for headphone amps used to be 120 Ohm output impedance. Some manufacturers still kinda use it, some don't. So some headphones are voiced to be used at 120 Ohms. This is why the DNA Sonett switches between very low impedance and 120 Ohms. Sowter, one of few manufacturers that makes dedicated output transformers for headphones, has models with multiple output taps so you can tune them to your headphones.
Look into the Damping Factor, as well. The lower the amp's output impedance and the higher the headphone impedance, the more control the amp has over the driver. However, there are some who argue that too high of a Damping Factor ruins the sound. I prefer a high Damping Factor, but look into the argument and decide for yourself.
For anyone who thinks that low impedance means something is easy to drive, read up on the Apogee Scintilla speaker. 1.2 Ohms of impedance and it is notorious for melting amps. Literally - lesser amps overheat and melt wires and components.
one step at a time!
