To be honest, an oval can be an inefficient design for woofers - for a given surface area ( think cone material) the circle maximizes the area (which leads directly to swept volume, SPL, efficiency) whereas a cone could use the same amount of material and end up with less area, and less volume output.

Also when moving, the oval woofer would experience an uneven friction and resistance from the air mass across its surface area, which could only lead to irregular movement or force applied against the cone mass - I'm sure it could be mitigated with consideration to concavity but that's a lot more work for less benefit already...

Edit: I know with car audio, oval woofers are an acceptable compromise to increase surface area in a restricted 2 dimensional area, ie, more width than length to work with.