That’s the area where we’re looking for the most clarity to start.
OK, got it! Take a look at the post below as the physics of low frequency measurements will require careful attention to settings. Can you first try the suggestions there and see if you can confirm the automated low frequency stepped tone sweeps in loopback, and then introduce your DUT and see how things change.
But just to double check, it sounds like static (non-stepped) measurements at 1 kHz and low frequencies are making sense is that correct?