What you describe here has nothing to do with cables or DC power.
You hit a pad, and a sensor in the PercPad generates a velocity value and sets this to a numeric value (between 1 and 127 I assume). This value is now fixed as MIDI data. It could be that there is a setting within the PercPad that can adjust this, but changing the cables, or DC power, will NOT change anything.
If you are sending the MIDI data to a DAW or some other computer device that allows you to store (as a MIDI file) or just as data, then there might be some facility to manipulate the MIDI data, which might well include the ability to change all/some volume levels.
I don't know why the present sensitivity of the PercPad is not enough for you. Can you see the values created now? If you make the quietest note you might normally make, what Velocity does that show? If you make the loudest note you might ever make, what velocity is that? The first cannot be less than 1, The second cannot be more than 127. A sensitivity control cannot change that. If you change the data afterwards on the computer, you're still constrained by the 1 to 127 range.