WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
While you could probably use it for a headphone amp, you could only use it for one channel. The 6AU6 is a single pentode, while the 12AU7 is a dual triode. You would need two 6AU6's to build a stereo headphone amplifier.
There are plenty of online tube references where you can look your tube(s) up and locate their datasheets. One such example is TDSL (Google it !), which also offers reference application for PC with links to thousands of tubes.