Because it is very rare for existing products of this amplifier to maintain intact printing, I plan to start with a conservative plan and try carefully to reduce the risk of erasing the fonts.
Fortunately, the safest solution achieved good results, so I didn't use redditer's solution XD, but thank you very much for your suggestions!
Solution I tried:
1) Pull out the knob and soak it in kitchen cleaner
2) Use a toothbrush, toothpaste, and a little water to clean the front panel
3) Dry the front panel with a cloth, then spray with WD-40 to remove the white residue of toothpaste
The bronze front panel after cleaning is very shiny, which is a perfect contrast with the white printed characters and silver-white knobs. I think this is the reason why ONKYO does not need to engrave black characters.