Oval cones

a quick search revealed:

http://www.carstereo.com/help/speakers.cfm

"Do round speakers sound better than oval-shaped speakers (i.e. 6x9's)? The answer is yes for most practical purposes. A round cone is more rigid than an oval-shaped one, so at higher levels, an oval-shaped speaker will distort more. The reason why there are oval-shaped speakers is because of rear deck space considerations by manufacturers. An advantage of a 6x9 speaker over a 6-inch speaker is that it has a bigger area, so it will move higher air volume, producing more bass."

not exactly sure how much detail you want...

My guess is you were looking for like combing effects, interference, dispersion issues, etc...?