None of that will work, if anything, itll make it worse, especially this part

Set HIGH to: 10-15hz +4db (play around with this to suit your hearing level)
What NightKids is after is the opposite of what youre suggesting. He wants the tweeters turned down because its too harsh, but by lifting the HIGH up its only going to become louder and even more harsh.

As for the high pass filter, and sub settings, this isnt going to do anything to lower the tweeters.

About all that you suggested that would work is adding an inline capacitor.

However, this isnt needed. That model of headunit has a 5 band Graphic EQ, (which is more indepth that just high/med/low) so all that he needs to do is simply adjust the 2 top end bands down by a couple of db until he finds it sounding nicer.