If I remember correctly, this is done by holding down the MENU button to access a deeper menu.
I'll have a play with my own car to confirm, but it's definitely possible because I have my default volume set to 6.
OK... Press and hold MENU until the hidden menu appears. Then four short presses on the MENU button until 'max on-volume' appears. Use the right-hand rotary knob to adjust the setting. Job done.