I have to say from my experience, the Dulciana in any form is the most useless stop in the whole organ, perhaps apart from the tubby Bourdons seen and heard everywhere today....
I have a actual Dulciana rank in my possesion, some may cry to hear this, but it is being re-voiced in my organ to function as a celeste for the Salicional...
If we were to look hard enough, we would definitely see that the first diminuative of the Diapason of unison tone should definitely be sufficient large to being second in power. If your main Diapason is going to be a 42-46 scale, for goodness sake don't make the Dulciana a 56!!!! For a 42-46 regular Diapason, the best solution would be to make the Dulciana according to the 48-50 range, and if it's still to loud, place it on a seperate windchest with a lower pressure, for just one stop, it's not that hard. Then, and only then, would the Dulciana be fully appreciated and used.....
The main thing that characterizes the almost inaudible Dulciana of these days, is that back when it was invented, the Great Organ division often had atleast 2 Diapasons, often more. The second Diapason obviously had to be quieter than the first, and in most cases due to inferior voicing, the second was made much too quiet in comparison to the first. The Dulciana was then meant to be even quieter than the second, which led it to no use, since the primary goal of the Great Division is in leading the congregation in singing in church organs. The need for a quiet stop (as quiet as this was) was certainly not that apparent, if one needed to accompany a small group or a choir, they would go to the Swell or Choir if you had one and use a Soft Flute/Principal chorus.......
~Alex