So the issue is narrowed down to, will the same cable (i.e. Kubala Sosna Emotion) in XLR version sound better than Kubala RCA (fitted with XLR adaptors) all else being equal? I have to possibility to try the Kubala XLR version.I think that you should check with the manufacturer to see if the rca cables are designed with the same internal configuration as the xlr cables. That may or may not be the case. If it is, try to arrange to have them re-terminated, so that you don't have to use adapters. If it is not the case, I agree with the preceding suggestions that the best course is likely to be investing in balanced cables, although of course it would be preferable to first do a comparison test (which you indicated is possible).
Many thanks.
To realize the benefits of a balanced interface, the cable has to have the "hot" and "cold" signal conductors designed symmetrically. Such as a twisted pair of conductors, each of which conducts one of the two signals, with the twisted pair being contained within an overall shield together with a conductor for the ground connection.
If the rca cable is designed such that the signal and return conductor are asymmetrical, such as a coaxial arrangement, noise pickup in the two conductors will not be identical, and will therefore not be rejected (or will only be partially rejected) by the differential receiver stage at the destination end of the cable, even if the cable is re-terminated.
Also, as Q indicated, nearly all adapters tie the "cold" signal (pin 3 on most USA equipment) to ground (pin 1). Not all output driver designs can work properly under that condition (although the majority of high quality designs probably can). See this thread for an example of a problem that was caused by using an xlr-to-rca adapter on an output.
And even if the driver stage can handle having its pin 3 output signal grounded, the resulting output impedance asymmetry with respect to the pin 2 output would severely degrade common mode noise rejection, as has been pointed out.
BTW, I agree with Q that a balanced driver stage (meaning one that has output impedances on both outputs which are the same), connected to a balanced input via a properly designed balanced cable, will result in identical rejection of common mode noise regardless of whether a balanced signal pair is present or not. The effect of having one of the two signals not present (i.e., at zero volts) will be a 6db reduction in signal-to-noise ratio and in sensitivity. And if the destination component is "fully balanced," meaning that it has a balanced internal signal path, some of the advantages of fully balanced design, such as cancellation of some forms of internally generated distortion and reduction of power supply related noise, will not be realized.
Regards,
-- Al