Though I don't own a QX-5, I do/have owned the Codex, QB-9 Twenty. Both perform well above their price point, IMHO.
As to why they use what they use, I would suggest contacting them directly.
Ariel or Ryan will probably respond. There are a bunch of Youtube videos on Youtube, too. Ayre is a really good company to deal with- at least from my experience.
Bob