5/6 Strings Double Neck Electric Bass and Guitar with Quilted Maple - China Electric Music Instruments and Musical Instruments Set price | Made-in-China.com
Bruce Sexauer's "The String Bass (.com)" offers a wonderful example of American Enginuity, Dalton Dillingham's 6 string "American Standard" double bass.
NS Design EU6 Electric Upright Bass - Amber | Sweetwater
6 string Double Bass; does it exist? | TalkBass.com
Double bass | Definition, Range, & Facts | Britannica
String Bass Instruments | 6 String, Double & Upright Basses For Sale
Jack Pimentel Electric Upright 6-String Bass Tobacco Burst | Reverb