Like you might expect, this is a forum and website devoted to computer and digital audio topics. You'll find lots of detailed information, both about the technology, and about particular products, here.
The largest and busiest forum for discussing headphones, headphone amps, and anything to do with headphones. Discussions include everything headphone - including both commercial products, reviews, and DIY projects and instructions.
An interesting online magazine about tubes and tube equipment. Includes lots of informative articles, including many about tube audio theory, and tube circuits. Also includes lots of info about tube-based DIY projects.