Of the switches mentioned, the Van Alstine is the only one supporting level matching. Without level matching there simply is no way a comparison can be valid.
As for colorations, any the switch might potentially introduce would be the same across the board, so effectively cancelled out when used for comparison purposes.
FWIW I used the Advent Speaker Comparator switch during my career, and installed the Audio Authority component switching system in the Definitive Audio showrooms in Seattle. Both of these were level matching devices, and extensie time was spent ensuring that the level match (set with pink noise and a sound level meter) was as close as possible. In both cases what was revealed was far greater than anything potentially lost, especially with speakers and speaker/amp pairing.