If there is an audible difference, something is defective.
@kr4 I'd put it the other way 'round. We audiophiles are very used to hearing interconnect cable differences. But recording engineers, who use balanced lines in a studio, are not. When the balanced line system is set up properly (as usually seen in a studio) the 'sound' of different interconnect cables goes away.
Having done many auditions of this difference over the last 40 years I can say that RCAs usually don't sound as good as a result and nothing is defective; this providing that the balanced line equipment actually supports the balanced standards (such as AES48).
"Fully Balanced Amplifier" means that balanced input section (instrumentation amp or transformer) is followed by two separate amplifiers, each driving one speaker terminal.
@kijanki Could you explain what is meant by 'two separate amplifiers'? Do you mean two single-ended amplifier circuits or two push pull amplifier circuits, or something else?