When the guitar was commissioned 16 years ago, I intended for the 7th to be either A or B, ie. a fourth or fifth below low E. For this piece I saw a need for a G and D note in the bass so I just tuned it down to G, and used the 7th fret harmonic for the D even though it's in the wrong octave. The string is a Savarez 5209 SI B9 that I buy in bulk from http://www.stringsbymail.com
The guitar is made by a local luthier, Jordan McConnell: http://www.mcconnellguitars.com/
Yes, this means you need proper speakers or headphones to hear the lowest notes. Just like a piano. I dig that.