Tillman buffer in a cable

[jubal81]

I'm going to build one of these phantom-powered JFET buffer cables.
http://www.till.com/articles/PreampCable/index.html

He says in the article that, in the future, he's run it at a higher voltage to get more headroom for humbuckers/high-gain pickups. The voltage part is easy (I have a few road rage boards). If I regulate to 15V, what value resistors should I use?