Well I know that when I spoke with Doug Dale of Coda, he explained how they work with their other products. He said that the companies will tell them what features are important to them, then Coda will design and build the amps/preamps to suit their tastes.
I've owned many Coda pieces over the years, great stuff. These guys know what they are doing, and would be a much bigger name if they didn't have such a disdain for the marketing type of people. Coda was founded by 3 ex-Threshold engineers. They like designing and building amps. They don't like the business end of the industry though. They would rather design and build amps for other companies than go to shows, schmooze with the reviewers, etc.