I thought the same thing, but the ZT Lunchbox has three prongs.

As I mentioned in the original post, only my THREE BL equipped guitars are doing this.  I should clarify that they are custom built guitars - all hand-built - while the 540RHH and N4 are stock factory guitars.  Neither of which produce this hum.

Indeed, it's perplexing.  My BL guitars are cleaner than anything when plugged into traditional amps.