atmasphere, I'm thinking about what you said, "My surmise about what is happening here is that the connection is what is important- it would be interesting to try just running dual connectors into a single cable (sort of the opposite of a Y adapter) and see what happens."
I'm wondering what you mean. Are you simply suggesting use of Y cables for XLR applications versus splitters? That has already been done, and I have reported on it. I have been using a few different brands of "reducing" Y cable for XLR in order to do Schroeder Method with them. There is a very noticeable difference in sound quality between the Y cables, as there is between splitters and Y cables. The highest quality Y cable I have used to date for splitting and then reducing XLR interconnects is from Audio Sensibility. I just finished a brief review of them which should be published soon.