Nelson Pass makes himself extremely accessible. That's why I decided to build one of his amps. Between all the articles he's written, his Burning Amp lectures, and the wealth of information out at DIYaudio in the Pass section, there's a lot of information. He's put enough information out there to clone a Super Symmetry circuit, and some have. He generally doesn't care about that kind of stuff so long as somebody isn't building it for profit. I've never heard the XA25, but it's architecture is similar to my F5, and I've read comparisons between the two to know that the XA25 is a better, more refined version of an F5. Then again, my F5 is more powerful