Hi Grant,
One of the limiting factors with a compression driver is Xmax at the lower frequencies. A 4" diaphragm has 177% more cone (diaphragm) area than a 3" and can therefore produce more SPL especially at the lower frequencies we are talking about.
Normally a 3" diaphragm has better VHF performance, but in this case (RCF 850/840 Vs 950/940) the 4" diaphragm seems just as good if not a little better.
The double 10 design we are talking about has quite reasonable SPL capabilites, probably a bit more an RCF TT5a, TT4a etc.
I have not measured the ND840 or ND950 so I'm guided by what RCF are doing with the speakers above which all use a variant of the ND950, only the TT2a, 22a use a 3" diaphragm driver like the ND840. The double 10 is noticably more powerful than these 12' speakers.
