I'll be curious to know what you settle on and what you think of the results. Long runs are what XLR cables were made for. Nevertheless, a long cable run should be more revealing of differences between cables than a much shorter run. I think these long lengths are how cables should be compared. At 50', a double blind test should be easy to pass, right? If you're struggling to hear a difference at 50', it's going to be a much bigger struggle to hear a difference at 5'.
There should be cable manufacturers jumping at the opportunity to loan you the long cables in this situation so you can compare and hear the differences that are going to be obvious.