Dedicated line -Ground wire, what size did you use

Need your advise. I pulled 10 dedicated line with a 10mm 2core shielded romex wire connected to porter port outlets, star grounded with a copper pipe driven into my backyard ( dedicated room on 1st floor). After using it for a couple months and with some feedback from friends we realized that the upper frequency is slightly harsh and with very slight noise. I decided to change the ground wire ( the shielding of the romex wire) by cutting the ground wire off and replace it with a 1.5mm 7strands copper wire. To my surprise, it make my system much more muted with exarbated bass ie sounded much bassier. I heard more bass now than the midrange. Its like seating 10 rows further away from the earlier set up soundstaging. I have another friend who did what I did as well and like his system much better now claiming less sibilance, and less fatigue sounding. He experimented with 2.5mm ground wire and find that it is harsher with less bass emphasis. For my system which is darker sounding I thk 2.5mm ground wire maybe better. Will probably experiment with 2.5mm wire after my new MCB burn in. Wonder whether that will improve the sound? By the way I change MEM MCB and ELCB of the DB box to Hager which seems to be beter quality. Any feedback and opinion is appreciated. Thks
Are you saying the dedicated lines are only grounded to the pipe in your backyard ? This is dangerous for two reasons. First, such a setup is highly prone to a lighting strike. Second, should a fault develop in the wiring, the circuit breakers will not trip.

In order for a circuit breaker to trip when a fault is present, there has to be a ground path from the branch circuit, all the way back to the utility company transformer.

When a supplementary grounding electrode (which is what you have) is installed, it has to be bonded to the previously existing grounding electrode. It also sounds like the new ground rod you installed is too far away from the pre-existing ground rod.

Branch circuit ground wire should be the same gauge as the current-carrying conductors. What country are you in ? In America we speak American Wire Gauge (AWG - #8, #10, #12, #14, etc.) not millimeters.
Dangerous? circuit breaker wont trip? I dont understand why it shldnt. I m from Malaysia. I have 3 phase wiring. I pulled 1 phase all the way after the meter box to my dedicated room abt 150ft away using a thick romex armored cable solely for my hifi equipment
New ground rod too far away? I have no choice as my dedicated room is located on the 1st floor. Probably grounded about 60-70ft away
1.5mm is equivalent to 15awg and 2.5mm around 10 awg. Refer to this link
I am not familiar with Malaysian electrical codes, but electricity is electricity regardless of where in the World you are, and behaves in the same way. At 150 feet from the meter box, instead of running multiple branch circuits, you could have run a single large feeder to a subpanel in your dedicated room, and from there, run individual circuits.

The earth is not a sponge to absorb electricity, it has a relatively high impedance. If you have a ground fault (short circuit) in the wiring or connected equipment that energizes metallic surfaces, the fault current will look for the easiest route back to its source, the power company transformer.

When you have a separate rod as the grounding means, the earth's impedance is not low enough to pass the amount of current that will cause the circuit breaker to trip and the ground fault will remain, posing a continuing shock hazard.

When a circuit is properly grounded at the main electrical panel/meter box, any ground fault current will flow back to the power company transformer at high amperage, because there is a direct connection between the two (low-impedance path), cause the circuit breaker to trip, and eliminate the shock hazard. Grounding at the main electrical panel/meter box is primarily to stabilize the electrical supply voltage. The power company transformer is also grounded at the pole.

Now, regarding lightning. When you connect your system to a separate ground rod that is not bonded to the main grounding electrode, any lightning strike that occurs between the two grounds, will cause lightning current to flow in the circuit because of the difference in impedance between the two grounds. Connecting the two grounds solves that problem because they will be at the same impedance (zero).

It is a common audiophile misconception that the earth is some sort of noise absorber and that a separate ground rod is the appropriate cure.

See the following link to a drawing I created that will hopefully show the relationship between the electrical source, your wiring/equipment, and ground fault current.

Thank you for the detail explanation and drawing and it seems so logical. Looks like a lot of us with dedicated room is doing it the wrong way for the sake of better quality sound at the expense of endangering our life. Wonder why my electrician did the grounding for my dedicated room in this manner. Thanks

Clarification:Malaysia electrical codes.
The power company incoming wires is only live and neutral wires and developer provide the grounding electrode. Thanks
I did some resarch on the Malaysian electrical system. It is the same as United Kingdom. North America also has incoming live and neutral from the power company, and the electrician installs the grounding electrode. The same grounding rules apply in both cases. The power company transformer (the power source) is grounded at its location.

For circuit breakers to trip when there is a ground fault, the current has to be able to get back to the source (i.e, the power company transformer) at a rate that is high enough to cause the breaker to trip. The resistance of the earth is too high to accomplish this. The breaker will just "see" the ground fault current as a normal (high resistance) load and not trip. That is why any supplementary ground rods must be bonded to the main grounding electrode (which is also connected to the incoming neutral, which is also grounded at the source (transformer). I'm glad that you understood what I was trying to say.