Thinking about installing a third pair of speakers under the dash grill (6x9 under package tray, 4" lower-front door panel). Want to fill the car with more sound and something from the front of me and above the door speakers would do so. Very little stereo from the dash so more like a center channel. I get good stereo from the 4" in the lower door panel. The system has a filter for the door panel speakers to block frequencies below 400hz Will test with the 4" two-way door speakers under the dash grill. If the results are good, will get either a pair of 3 1/2" or tweeters. With the 4" two-way in the lower door, I think tweeters would be a good addition but will wait till the sound test. Would power the dash speakers with the same amp that powers the door speakers. (Package tray speakers have a separate amp). Series wiring two speakers (both 4 ohms) on the same channel will double impedance but power drop is not a corresponding half reduction. Series is safer then parallel. Each amp channel is 35rms at 0.5% thd. Tweeters I looked at would have db of 93-97 at 2 watts so not much power needed for volume A fader would be needed to balance the door and dash speakers. Don't know how this would be done but since it is in series would a fader be needed for the left pair and one for the right pair? Where and how to install a fader in the wiring?