I also have a fully balanced tube preamp (Calypso). I use the RCA preamp outputs to my amps as it preserves the tube character of my preamp. As said in other thread, the balanced outputs remove the even harmonics of the tubes which make the sound more flat and less tube like. For the input, the sound of the RCA and XLR is closer and it is not as clear cut. What I would recommend is that you buy some cheap balanced cables and try them on the inputs and outputs. Listen to the difference between them and your fine RCA cables. You should be able to tell if you like XLR on outputs and inputs with your own ears. After that you can decide if spending the money for one or more sets of fine XLR cables are worth your effort.
Bob
Bob