LOL at the eHow article...it sounds so simple!
The DIY forum in HeadFi and the HeadWize projects are really good sources. Use the search function and dig around a bit--you'll find some neat stuff! Start by looking up "Millet Hybrid." That's an oft-suggested starter build.
You'll need an input buffer for your bass, going into your tube headphone amp circuit. It probably would be nice to add a switch between a stereo source input and your bass input, so you could use the amp for passive listening...or better yet, do a simple mix buss so you can practice with your mp3 player or your stereo. 'Twould be a killer practice setup!
The "instrument" sub-forum at diyAudio is a good place to get ideas for a preamp:
Musical Instruments - diyAudio
If you register and post there, do your research and phrase your question carefully. They're kind of fanatics over there...
Also check out Rod Elliot's pages - lots of neat projects and useful whitepapers:
Elliott Sound Products - The Audio Pages (Main Index)
EDIT: As far as myself, I've built a Cmoy and a balanced a47 headphone amp, but haven't done anything with tubes as of yet. I am gearing up for a "take-no-prisoners" solid-state bass preamp build, though.
