Can you build a transparent XLR selector box?


I know that the answer is yes, as in every professional recording studio employs patch bays.

But have you personally tried one in your hifi signal path and were able to notice any effect on your sound?
yukispier
I only ever heard one that didn't sound terrible, and that was the Manley skipjack.

RCA only though. 
I had the rca version of the Mapletree LR-1 line router custom built for me and it was excellent.  They also custom build the LR-1 Pro line router which is the XLR version.  

https://www.mapletreeaudio.com/