He does use rather heavy gauge. Backwoodsman and I both know his guitar mechanic who, when I asked him to lower the action on a guitar I have told me Martin would want it raising if it were his....

He certainly used D’Addario when we had him last at Roots. One of my chores is to “leave the dressing rooms as we found them.” Incidentally, chatting during a sound check, Martin advised me, and by extension anyone who uses a thumb pick and just the pads of fingers to pick to use very heavy bass strings and lighter treble. I use .012 to .060. (Made up sets by Newtone.). If I have to buy off the shelf, most string manufacturers these days do what they call Bluegrass sets, .012 to .058.

Tunings? I’ve yet to hear him sing two songs consecutively in the same tuning....