Hey that looks like a cool project...... Can you post a schematic? with voltages you are getting ?
jr_tech has a point about matching the source and load.... look at this:
http://en.wikipedia.org/wiki/Impedance_matching
What you want is for the output device to "see" a speaker or earphone load, And for the next stage to see its natural input resistance, or "impedance"
The circuit you have, the output device will be very unhappy whenever the volume control gets turned down under 1000 ohms. (just because I have seen headsets in the range of 1K to 8 ohms) The input of the tube circuit would like to see around 50Kohms or more I would imagine....
I found the circuit, you should try to get it working with the volume control at mid point, so it don't cut the tube off....
Remember maximum power transfer will only be 50% not that you will need that......