XLR cables were originally meant to be used for low impedance microphones (or other low signal strenth audio sources) under conditions where RF or EM interference can exist, such as in recording studios. They are most useful when there are lots of different wires and cables all over the place, and when studio technicians can't be bothered with tracking down sources of interference.
The signal from an amplifier to a speaker is orders of magnitude stronger than what is carried in any RCA or XLR interconnect. It is likely to be strong enough to be immune to the types of interference that are trouble for low strength signal carried by audio interconnects. So, even if your preamp and amps are designed to use XLR connections, it is a solution to a problem you don't have.
Real interference is a problem with a real solution,
audiophile upgraditis nervosa is a problem that no cable will cure
. Stick to the real problems.