XLR=Balanced
RCA= Single ended
Except that some amps and/or pre-amps have XLR connectors and do not have balanced circuit design. Basically its just an inboard adapter. Balanced cables have better noise rejection on long runs. Truly balanced circuitry in an amp or pre-amp will have a lower noise floor. Some companies (BAT for example, use all balanced circuitry) Others just accept both types of connectors.
RCA= Single ended
Except that some amps and/or pre-amps have XLR connectors and do not have balanced circuit design. Basically its just an inboard adapter. Balanced cables have better noise rejection on long runs. Truly balanced circuitry in an amp or pre-amp will have a lower noise floor. Some companies (BAT for example, use all balanced circuitry) Others just accept both types of connectors.