It was a simple jumper move from position A to position B. It wasn't called out in the owners manual. The unit sounds oh so fantastic, super quiet, like dead quiet. If you didn't know better, you'd think it was solid state. I have been rolling some tubes 12AX7/12AT7 using some early 50s Telefunken and Amperex. it responds very well to tube changes. I think i prefer the Amperex, it's a nice in-between the clean Telefunken, and the oh so lush Mullards. I haven't tried the Mullards yet, but no rush, just enjoying the unit.
I purchased the unit from David Michael Audio, worked with David, he was extremely responsive and had a great purchase and support experience from him. I'd for sure go back to him!