Pete Millet's site is a pretty good resource, he's posted a lot of different designs ranging from preamps to power amps to headphone amps. His "Starving Student" design was my first foray into building a tube amp about a decade ago, though that one is really a tube/transistor hybrid and it's designed for headphones but I used it as both a speaker and headphone amp.

http://www.pmillett.com/