50Hz and lower is non-directional sounding. Anything above that generally is. That means, all tones above 50Hz for most people, can be located easily. If the tone is under 50Hz, you cannot tell where it is coming from. For all you know that 30Hz tone is coming from up under the dash...lol I can clearly tell where my subs are when my filter is set at 80Hz. It's when I drop it to 50Hz I cannot tell. This is the standard.

With that being said, I have Focal K2P midbass's for rearfill in my rear doors and they are filtered to send out 60-350Hz and when i turn them on I can def tell where there are located.

As for woofer placement. I spent the most time aiming my woofers and building the mounting plates for them to angle towards me. There was a huge difference in sound. Instead of the wavelenghts bouncing from one woofer to the other(which creates reverbs and dead sound in the middle) They were aimed up at an agle towards my listening plane. Each mid-woofer is on time alignment just like my tweets.
Woofers in the doors with tweets above the knee is a good clear listening environment even on the highway. The problem with door speakers is when the tweeters become blocked and the sound is muffled. Make your life easy...Get 2-way components, matching rear door woofers only and call it a day.
3-ways are very expensive. if your buying budget priced 3-way component sets, STOP. You'd be much better off buying a quality 2-way Component set of the same money. You'll get waaaay more bang for your buck. If you don't have $2k to spend for 3-way Comp's, quality amp, and a sound processor, your wasting your time!