Hi All,
       his is a subject I'm addicted to , but which actually drives me nuts at the same time!!! I learnt 12-string off Pete Seeger's Leadbelly 12-string instruction manual and 2 record set. Pete alway's tunes 4 steps down to C. Leadbelly tuned down as much as five steps and Leo Kottke also tuned to C and C# sometimes even down to B. This seems to have come about as a result of the Stella type guitars with long scales and tailpieces, combined with heavy strings. For years I struggled with lovely 12-strings ruining them with all the experimentation with heavy to very light.The fact is most 12-strings work better with ultra light strings at concert pitch. I solved that problem by getting Bruce Taylor who builds Pete Seeger's Guitars based on the Stanley Francis guitars (not Bob!!!)to build me a Pete Seeger model 12-string with the heart shaped sound hole , but were also DESIGNED for low tuning. I am ashamed to say I have so many guitar banjos etc I have lost count and having an undertstanding wife I bought recently a Gibson B45 with a tailpiece that only works properly with light strings 10-47 or 50. That is why Leo Kottke had to abandon them for his style. I also like the clarity of concert pitch. The Seeger guitar uses a La Bella custom magde set y13-070!!! with wound 018's for the second pair.Harry Lewman the Leadbelly expert also has had special guitars and La Bella made for him. There unfortunately is your answer. If you want to tune down with heavy strings (light just go out of tune at low pitch and also buzz) get a guita made for the job After more tha thirty years playing 12-string and struggling with this problem I know!!
          Regards
            Jonathan Klass
             South Africa