Is grounding with RCA is safe?

It’s safe.
Now, there are two complications:

1. as noted you could create a ground loop, but from your report of improved sound, you didn’t, so don;t worry about it.
2. Safety, and effectiveness, depend on how your component is grounded internally. The outside RCA connector is signal ground, for sure. But signal ground and chassis/safety ground may or may not be the same.

• they may be connected internally
• they may be independent internally
• there may be a switch (many of my designs had one to eliminate ground loops)
• They may be connected via a resistor, sometimes around 1-meg-ohm

So, while it won't injure you (its grounded after all), it might or might not provide an improvement.

G

I disagree that grounding options could reduce the accuracy of the system, **unless** they also created a noticeable hum.  no hum, no foul.
The key to grounding is to have ONE connection to ground - and everything else grounded to THAT.  Typically that one point is the preamp.
What I would do is

a) find out if your preamp connects signal and chassis grounds. Ideally float the signal.  Find out the same for your other components.
b) either  b.1) don;t ground any other components or b.2) ground them all to a single power strip/conditioner plugged into a single outlet.
never, ever plug some components into outlet #1 and others into outlet #2. That's the recipe for a ground loop.
Grounding chassis of components to actual earth (3rd plug) is always both safe and good at reducing radiated noise. The problem only occurs if the manufacturer connects chassis to signal grounds.
but I'll re-emphasis --- if it soudns better and has less noise, it almost cannot be creating a loop.
G