One of the factors that has not been discussed above is the design of the actual connectors
- XLR conncetors convey the signal via low mass pins - this works much better for the delicate signals involved
- RCA connectors generally tend to have a relatively low mass signal pin but a much higher mass neutral barrel.
- This can degrade performance, so an XLR cable may sound better
- However, more recent RCA designs use low mass pins and as a result they can perform better than their XLR counterparts.
- also, an XLR cable, having effectively two signal wiires will suffer more distortion that is generated by the insulation on the wires
- i.e. wires using an insulation with a lower value dielectric constant provide significantly better details, clarity and imaging
If you were to take two very good cables from a single manufacturer - I doubt if you would notice any difference
So, it is not as simple as XLR vs. RCA - it comes down to the cable design i.e.
- metals used in the wires
- type of insulation used
- type of connectors used
- the geometry of the cable
For more details on this take a look at
https://forum.audiogon.com/discussions/what-i-have-found-makes-a-good-cable
Regards - Steve