Is grounding with RCA is safe?

Don't do it! You are creating a ground loop! Your gear is best grounded through each component's ac cable!

The only way that occurs to me in which that might be hazardous to the equipment is in the very unlikely event that an insulation failure develops within a component, resulting in the AC line voltage being applied to the chassis of the component and from there via the AC safety ground connection and the wires you have installed to the internal circuit grounds of the components, during the brief amount of time it would take the breaker to trip.

However, as Roberjerman indicated you are making the system very susceptible to ground loop issues. In effect what you are most likely doing (depending on the designs of the components) is connecting the internal circuit ground of each component to the chassis of that component and to the chassis of every other component in the system. Thereby in effect changing the designs of the components, as well as the ground paths between them.

While you have found the results of those changes to be subjectively preferable to you, you have therefore most likely compromised the accuracy of the system.

Regards,
-- Al

I see. Thank you all for your inputs. 

I searched online and learn that ground loops can destroy audio gears in long run. Is it true?

I still have a question regarding your reply: 
"In effect what you are most likely doing (depending on the designs of the components) is connecting the internal circuit ground of each component to the chassis of that component and to the chassis of every other component in the system"

I check the ground box from Entreq. It looks like to me that they do the same thing.

Could you help shed some light on this?

I have to apologize for my ignorant questions because I am a dumb in electricial area.

Thank you.