These last two posts have raised a question for me. My BPIII will be going for a tune up later this year. It has been played for twenty years without one so it's about time (I can almost hear the resident fettlers tutting). As far as I am aware it is tuned LMM+. How different would it sound if it were to be retuned as LM-M+ ? The box is swing tuned.
I have played it with several fiddle players over the years. Nobody has complained but I don't think any of us has perfect pitch.
If done properly, Dedic tuning on a BPIII to L M- M+ will work fine and you should get a good rich in-tune sound with all three voices sounding, and also M- M+ will sound good too. If the basses and chords are tuned to 0 cents offset, the treble and bass ends should sound nice together.
The problems come when you have a LMM box where the M reeds are are individually controlled by their own stops, e.g. a Saltarelle Nuage or Serenellini 233. Dedic tuning would work fine so long as M- M+ or L M- M+ reeds are selected, but pairing L M- or L M+ will sound horribly out of tune. With these instruments it's best to tune them LMM+. That way you can have a nice sounding LM 'bandoneon' setting when needed.
Which is why I only ever suggest it as the best choice for 2-reed MM boxes with no RH stops... ;-)