Tube Headphone Amp. Getting Started.

I'm just getting in the audiophile game and since I do most
of my listening using headphones, I think getting a good
non-fatiguing tube headphone amp would be a good idea. I
know some DIYers who could build something for me, but am
frustrated by my ignorance - I don't know what kinds of
questions to ask.

What kind of things should be considered (besides obviously,
just listen to the thing)? I figure the process of learning
is probably best served by trying to build a simple amp
myself. A DIY kit? Background reading?

Has anyone been down a similar path? I'm willing to do the
grunt work. Please just get me started with some pointers.