https://hifisonix.com/
The DIY site of the creator of Ovation amplifiers. Has very good projects with detailed build guides and a link to purchase boards. Also a number of detailed articles with those on grounding especially useful.
http://www.markhennessy.co.uk/audio.htm
Documents projects with extremely detailed guides.
https://theslowdiyer.wordpress.com/
Documents his projects, and often makes the files available.
https://emumannen.blogspot.com/
Very nice homepage with detailed builds