You still get benefit of common mode noise rejection using balanced cables even if amplifier is not fully differential. You also get benefit of better, safer locking connectors. Balanced cables are better, IMHO, but it is matter of your environment (amount of electrical interference) and design of an amp. If amp is designed such way that extra output and input stage can compromise the signal, as Almarg explained recently, then you might get better results with single ended (RCA) cables. Balanced/differential input is often converted to single ended signal (ground referenced) by input transformer. This transformer does not improve sound and is known to produce harmonic distortion at low frequencies. Small distortions at low frequency are not very audible (many great amps have transformers at the input) but still it is a compromise.
Fully balanced design is better but costs more. Common mode noise rejection improves a bit, even harmonics produced by an amp are canceled, power supply gets balanced load (no big ground return current in power amp), each half requires half of gain and the same output power can be obtained from twice smaller supply voltage (bridged output). There might be some issues of gain or phase shift matching so it would be interesting to hear from the amp designers.
Fully balanced design is better but costs more. Common mode noise rejection improves a bit, even harmonics produced by an amp are canceled, power supply gets balanced load (no big ground return current in power amp), each half requires half of gain and the same output power can be obtained from twice smaller supply voltage (bridged output). There might be some issues of gain or phase shift matching so it would be interesting to hear from the amp designers.