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.

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?

The internal circuit grounds of most components are connected to the ground sleeve of their RCA connectors. And since the components apparently have three-prong power plugs, the safety ground prong on those plugs is presumably connected to the chassis of the corresponding component.

So since you have connected the ground sleeve of an RCA connector to the safety ground pin on the power plug of each component, and the safety grounds of all of the components are connected together via the outlets and the associated AC wiring, you have most likely connected both the circuit grounds and the chassis of all of the components together.

I have no knowledge of Entreq products, but if in fact they do the same thing I personally would not want to use them.

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

I can’t envision a means by which a ground loop could cause damage, other than in the very unlikely scenario I described in my previous post. A ground loop could, of course, have adverse sonic consequences, and degrade the accuracy of the system. Which may or may not be subjectively preferable, however.

Regards,
-- Al

I wouldn't say it is unsafe, per se, but many designers deliberately lift the ground to prevent noise issues. Seeing a 100 Ohm resistor between the chasis, which is directly attached to the AC ground, and the signal ground is quite common. 
The quietest systems I've ever heard completely isolated the signal ground from the AC ground, preventing ground loops altogether.

I would not encourage anyone to start hacking this way.