Series or Parallel for tweets/ door speakers

FWIW, drastic cuts at 315Hz and 1.25kHz, with more subtle cuts at 500, 800, 2k, and 3.15k really helped clear up my vocals in my truck. But I will say that they are not equal cuts between left and right. The right is cut more at lower frequencies and the left is cut more at higher frequencies. I'm pretty sure it's the result of speaker locations. I use 1/3 octave filtered pink noise tracks and found them very useful in centering up the frequencies and identifying nasty peaks.

Generally, I found that cutting 200 and 315 (and a little of 125) gets rid of the chesty sounds. Cutting 500 and 800 gets rid of the nasally, hollow sound. Cutting 1.25k and 2k gets rid of the really tingy sound at high volume - our ears are really sensitive to these frequencies so too much will make you shrill. 5k and 8k (and a little of 12k) is where the hiss is at.