The amplifier noise will be the same as long as the meter does not add any instabilities. For simple null tests I run it at absurd gains (10,000,000) so I don't have to think about the noise from the digitizer (DVM).
I started a test of 6 nanovoltmeters this afternoon. I'm testing the noise of a A10, A22, A23, K181, K182 and a HP34420A. The EM preamps are running at a gain of 1000. A K2000 is scanning and digitizing the outputs. If the noise is an issue I'll change the gain to 10,000 and try it again.
For the A10 and A23 the factory recommends a minimum gain of 1500 (1500 ohm feedback resistor). This minimizes issues with the output voltage swing limit of +-3V and the +-2ma output limit. With the Keithly 1801 preamp (A10) they could use a gain of 1000 because they were on a +-2V meter range. The output current limitation caused me issues when I was running at a gain of 1000 with a 3mv offset. I did not realize I had exceeded the 2ma limit. The amplifier does not clip it seems to do a gain compression.
The A22 module uses a 10 ohm internal resistor so it can work with a 20mv input differential voltage.
In the Keithley 1801 datasheet or manual there is a block diagram of the EM preamp operation. It chops the signal at 488Hz and amplifies it with a transformer. There has been at least one disassembly of the EM Electronics N1 or N2 Nanovoltmeters on the forms.
Many years ago I got a badly damaged A10 amplifier and did some reverse engineering. Including the potted mu metal box, which was a real pain in the ass to open. I hope I can still find the documents. I don't promise that they are complete or correct, but it shouldn't be that difficult to fill in the gaps.
A10 circuit:
This had a partial schematic attached. I did a little Photoshop work on it and have attached it below.
Hopefully this is not to far off topic...