What I found with selectable imp MIT cables was as the imp was changed, the midrange was the most affected.... either becoming more or less recessed... subtle diffs are elsewhere... usually I had mine selected for high most often, as both the preamp and or amp were at higher imp than the sources, or preamps, output imp.
unless I got it all wrong all the time, you select for the downstream devices input imp. That info was based on using RCA ics.
With XLR it can get interesting, as there are now two switches for each cable. Playing with them both can provide all sorts of results, vs. the fac recommendations, which is to set both sw. to the same setting.
I found the MIT mag series a nice wire for sources.