Jack:
Is your Demeter a stereo preamp with a stereo tip, ring, sleeve, jack? If not, I believe that may be your problem. The DS5 converts your stereo into a mono signal. When using batteries, if you are plugging a stereo plug into a mono jack, the preamp is only seeing one side of the stereo signal. You need to either rewire the guitar for mono, or split the stereo signal into two mono plugs and plug into two channels or inputs on the preamp (if it has two channels or inputs).
Bill, tgo