I was told each unit sounded better using bal I/C but the amount of change was a surprise!! It was as if the S/E had been choking the system down!!
Such a huge performance improvement is unusual - perhaps you had a problem with noise from a ground loop with RCA. Did you carefully check signal levels - sometimes a higher signal or higher volume is normally immediately perceived as a sound improvement (particularly bass is what blossoms at higher volumes due to the way our hearing works...)
Certainly I highly recommend XLR as it means less hassle/problems - better sound more reliably and almost always a lower noise floor - however I would not claim XLR sounds dramatically better than a good working RCA connection - it should sound the same if gear is working properly and no noise/ground loop issues.