This may be due to the headphone output power.[size=12.727272033691406px]
In the past there were two types of CS-300X, one has 1000mW headphone[size=12.727272033691406px][/size]
output, and the other has 800mW for high efficiency headphones.[size=12.727272033691406px][/size]
[size=12.727272033691406px][/size]
Your CS-300X should be with 1000mW headphone output for low efficiency[size=12.727272033691406px][/size]
headphones, therefore, the noise shall be improved by reducing the[size=12.727272033691406px][/size]
headphone output power from 1000mW to 800mW.[size=12.727272033691406px][/size]
[size=12.727272033691406px][/size]
Please refer to the attached schematic diagram for CS-300X.[size=12.727272033691406px][/size]
There is 270 ohms resistor at the headphone out.[size=12.727272033691406px][/size]
By replacing this resistor by 1K ohms(30W) resistor, the headphone[size=12.727272033691406px][/size]
output will be reduced to 800mW.[/size]