I noticed that in the MB1 board (I/O balanced interface), the THAT1646 balanced line driver is fed directly from the wiper of the output level potentiometer, which is definitely not good practice. The THAT datasheet is very clear about it, the 1646 should be driven from a very low Z source.
The penalty is seriously degraded CMRR. It should not be a big issue in most cases, since the Prism is generally connected to an Optimod or similar, located very close, so there's no long cable runs. However, in a heavy RF environment, that could be a problem.