It seems that the main issue was a design flaw. The AD8336 has a preamp stage, and if the gain is too low there the noise performance of the whole thing is bad no matter what you choose as the "variable gain". The universal box has this preamp set to unity gain. As seen in the plot, if you extrapolate the "preamp gain = 4x" gain line up by a factor of four, and assume we were using a reasonable gain setting of ("G" = 5) on the dial---corresponding to V_gain = 0---the input noise is 100 nV/rHz. This is bush league.

Also, I don't see a single reference to any frequencies lower than ~100 kHz, and I suspect that the low-frequency behavior is not a priority for them. We should avoid this part.