12AX7 has 5 times as much gain as 12AU7, which means with a 12AX7 installed and the gain turned up you can get a fair amount of overdrive. Some people like this, some people don't. With a 12AU7 installed the pedal becomes completely clean with no overdrive, and the Gain control becomes more of a "saturation" control.
Yup that was it. I've been talking with Philip about doing this for over a year, and after we met in person at NAMM we chatted some more and decided it was time to finally (lol) make it happen. He's been using my pedals for years and we've chatted quite a bit, and he is the nicest person you'll ever meet. Super excited to work with him.