For those of you interested in the design and layout of these two fine models, some pictures of both the PRO78 and the XTR690 are available from the FCC website:
Compare with that of the pro-73 (similar to the 1783):
The 78 is definitely another animal all together. The layout has very little resemblance (if any) to the 73. The horn geometry has definitely been redesigned. It seems to be significantly longer than the 73's, which probably explains the increased sensitivities.
They really meant it when they said the 78 was re-engineered. 8)