The circuit allows for the different output tubes so I think he's just offering it as an option.
I'm on the fence with the AD1s...will make the decision once I get to hear them.
Supposedly, it's not hard to change the type of tubes...Craig just needs to change the tube sockets for the appropriate one so people can change it in the future if they want to.
Dang...end of week and no pics...hopefully, just a few more days.
I did hear a prototype and it did sound sweet

You could hear another level of detail/texture from the RS1s that I had brought at the time and that still was not the final version of the amp.