You seem to know that the distance between the string and the pickup is critical for volume. I think you still have a few choices with your instrument. On the setup side, see if the high strings are as low as the other strings. If not, see if you can adjust the bridge and nut to reduce the distance. I assume you have the truss adjusted where you want it. This is crucial for tapping instruments for volume and dynamic range. As an end run if you need to, just invest in a good equalizer. This is not the best choice, because it will affect all strings, not just the high ones. You can always ask Kevin Siebold for advice, since he made it after all.