I have a suggestion, but it is based on a two assumptions: (1) the difference, if any, between sources is minimal and (2) you are able to eliminate audible relay clicks (either by moving the gear to a separate room or behind a screen).
I would encourage you to include the following test, perhaps somewhere in the middle of the session: given the previously discussed 1/4 second factor, hit the mute button on your preamp, do not change sources, then hit the mute button again.
I would encourage you to include the following test, perhaps somewhere in the middle of the session: given the previously discussed 1/4 second factor, hit the mute button on your preamp, do not change sources, then hit the mute button again.