Hi
@Harry
Its a good question. There is actually a very good technical reason for it. The amplifier is fully balanced input. Balanced inputs are fully compatible with single ended RCA, however for best performance you have to follow correct implementation of the conversion from one to the other.
The correct way to do this is not to have RCA sockets on the amp itself. You should only terminate the balanced screen at the far (source) end like this;
View attachment 40703
Unfortunately RCA single ended connections have a fundamental flaw. The low signal conductor (screen) is often connected to the chassis of components and mains earth. This means that noise currents flow in the shield between components and modulate the signal. Its a pretty dumb idea really. Its the fundamental cause of the buzz when you have a "ground loop", but even if its not overtly noticeable as in that extreme circumstance, its still a source of unwanted noise.
In balanced connections the shield is a 3rd separate wire from the signal conductors. So if wired as above those noise currents dont flow in the signal conductors, even if the source is single ended RCA, and you maintain most of the benefit of balanced connections. If you put the RCA socket on the amp the noise currents still flow in the cable shield.