XLR connectors are normally used with balanced line cables; the standard for coaxial cables is RCA (in audio, anyhow). Balanced lines have no ground reference (the third pin is a shield, not a signal ground). These standards were originally developed with transformers at input and output, and that is still the best way to convert between them.
Transformers with good audio performance are inevitably expensive, so most audio gear now uses electronic approximations to give similar performance. Several different approximations are used, not always compatible with each other. Adapter cables usually just ground one line of a balanced line, which usually works, though the noise isolation of balanced lines is lost.
If a single ended output has a constant impedance, then a better approximation does that grounding with a matching resistor, restoring the noise isolation. That is what we did with the original BeePre. but Moreplay and BeePre 2 do not have constant output impedance. In BeePre 2, we add a cathode follower after the unbalanced output to obtain that constant impedance and thus retain the noise isolation. This is not possible with the Moreplay, so an adapter cable will work but loses the advantage.
At the input, an adapter will still work if the source device is truly balanced, i.e. it has no signal ground on either line. But if the "balanced" output of the source device, is actually just push-pull, i.e. both lines are referenced to signal ground, then a converter would short one phase, possibly damaging the performance. Push-pull outputs are common in modern gear, and often incorrectly called balanced, which adds some confusion to the topic!
There are some papers on the Jensen Transformer website which provide much more information. Here is a link to one of them:
https://www.jensen-transformers.com/wp-content/uploads/2014/08/an003.pdf
