From the result, it was confirmed that the power amplifier part of TPA 3255 is normal.
Also, it is unlikely that two OP amplifiers will be defective at the same time, so the OP amplifier will be normal.
If you have another op amp, try replacing it and testing it. If the noise disappears after replacing the OP amplifier, the cause is a defective OP amplifier. But that's unlikely.
The most suspected part of the results is the DSP circuit. However, this part requires a dedicated interface and control app to be installed on the PC.
Also, since the noise is constant regardless of the volume operation, it is judged to be normal between the RCA terminal and the input of the DSP chip.
The power supply is probably normal.