A good OM may not register on a meter as louder than a dread, but they project better than a dread. They "cut" through groups of other live instruments very well. They just do not have the swollen bass at the bottom end that a dread provides.
To my ear, dreads sound better to the player than someone sitting 10+ feet away. I mean when it comes to the fullness, to that bass that goes right into your torso and envelopes you as you are playing.
An OM never sounds as good to me when I am playing them as they do when I am out front, listening to someone else play it.
And those Authentic OMs have a balance that is even more impressive than normal OMs, in the way the notes pop out into the room and each string stays at the same plane as the others.
I have played a handful of the
18A and only one of the prototypes of the
28A. The 18As all had very stiff tops - but none more than the prototype I played. They required a bit more attack to get them to bloom. The 28A did not, although it did really like being played with more attack than required to get it to bloom.
But the 28A was the only one with the new lightly torrefied top Martin is now putting on all the Authentics.
I always say Madagascar is the rosewood for mahogany lovers, as it is less thick and smoky under the mids compared to some other rosewoods, and it has that very clear and clean chime up top unique to that species. But for sure it has more overtones than mahogany, up top and also spreading out from under the bottom end - just not as complex as Brazilian and not as dark and warm as Indian, but more of both than mahogany.
Otherwise, the string spacing is different (2-3/8" for the 18 and 2-5/16" for the 28) and the necks are different, but similar. The string spacing likely helps them feel less similar however than just the uniquely carved profile of each.