What you are doing is perfectly safe. The outside is ground anyway. Its real easy, with tube gear anyway, to look inside and see the one wire coming from the center of each RCA and going straight to the input selector. Only one wire. Don't take my word for it. Look and see. One wire. That's the pin, that's the signal, that's positive. That's why its insulated, and routed the way it is, away from noisy bits like transformers. That's why they lay them out the way they do.
Notice the outside of each RCA. They are not wired individually. They do not go to the input selector. On many amps they connect to a bus bar. Follow that wire, which notice is not insulated, it will at some point complete the circuit by being connected to ground. They may all come together at one point, I don't know, have only looked into and modded a few so there may be some exceptions.
When there are you can be sure its for sound quality not safety. Mainly the problem is the way electricity works, running a signal through a wire generates a magnetic field around the wire proportional to the signal. While at the same time every magnetic field crossing a wire induces a voltage in that wire proportional to the field. This is how electricity is generated, be it in a Koetsu cartridge or Hoover Dam, and this is how transformers work too. And antennas. Same deal.
So if you're hearing an improvement that kind of makes sense. What you're doing is providing a more direct route to ground taking that one small part of the problem out of the amp. A little less is a little less and if you hear it you hear it.
Count yourself lucky. The ground loop guys are right. Only thing they're missing is its a crap shoot. One guy does everything right, winds up with hum and hunting down a ground loop anyway. Another guy (that would be you) deliberately wires extra grounds which one would think ought to hum like crazy, and instead it actually sounds better. Go figure.
