1. I don't like the logarithmic variable gain. I like the dial on the box which indicates the number proportional to the gain. This means that the variable gain stage is to be an inverting amplifier with the potentiometer in the feedback path. I recommend it as far as AD829 does not get screwed up by the stray reactances of the potentiometer.

2. Let's put buffered test points before and after the excitation input. These test points should be connected to the panel connector. There should be a manual (or remote?) switch that disconnect the exc input such that we can avoid the noise injection through it.