IME, used cables require a short run-in period (depending on how long ago they were in use).
Consider that they were coiled, boxed, then maybe sat for awhile. They’ll need to physically settle into your setup. Cables present changes in sound during the break-in period.
This is exactly right. However, its not like going back to Day One all over again. You should have heard big changes the first hour or so. Did this happen? Usually then they'll continue to improve, but much more gradually. In other words just like normal break-in but faster and shorter. Are they doing this, still gradually improving, or have they hit the plateau?