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