Some point in the last 20 years, my wife bought the James Demeter / Preamp EQ 18 volt model for her guitar. After my preamp died, I went searching for the same model as my wife's. They are pretty hard to find but I located one and bought it. The outer gray box is the same but mine is a nine volt model as opposed to my wife's 18 volt box. Can anyone tell me what the difference is? 18-9=9, I get that part.