When the voice coil pushes up on a round cone, force is linear.
When it pushes up on an oval cone, force is not linear.
The 6" side of the 6x9 will flex less then the 9" sides.
From my experience, 6x9's, no matter the brand, do not last as long because of that.
If it's a stock system, headunit and speakers, it CAN be hard to find a speaker that will be the proper ohm rating.
In a Toyota Sienna Symphony, it was [censored] near impossible to find a nice 6.5 with a 2-2.5 ohm rating. Old Image Dynamics and such were the last I remember when I was in the field.
Needless to say, I dropped some 4ohm Macrom 6.5's in and they are not as loud as the rear anymore due to not pulling the same power as the rear.
Fade to front, and problem "fixed"... rear speakers pull the soundstage backwards anyway.
(If you are getting a sub, and you plan on replacing all four door speakers... Spluge on the fronts, forget the rears. The fronts when properly powered will fill the whole car with sound and not pull the sound stage to the rear.
I have about $12k of audio I was going to put into my Tacoma until it was keyed and paint never matched. So much for the shows and the system...
Front stage:
Phoenix Gold Ti Elites... Made by Morel.
4 9's
4 6.5's
2 5.25
4 1's
And all crossovers to go with.
This is just part of something that never happened: