Yeah its a small but significant improvement. Kind of like "oh, that's what their supposed to sound like "
A single 9 volt battery does drop down to 8v, 7v, 6v.... fairly quicker then we think.
So really after some hours of playing, the single 9 volt is at 8 volts maybe even 7.
With the 18 volt, you doubling your power.
The following is from the EMG power guide PDF instructions.
...but it allowed me to make room for my Forte so I can't be too sad about it! 
