No metal cutting, I'm talking about 6.5's that aren't actually 6.5's that will fit in a 6x8 opening with an adapter plate. No room for some huge beefy 6.5" semi sub/semi mid.

2010 Ford Focus SE, four door sedan, 6x8s in all four doors. Factory bass is halfway decent, just not very loud. I am beyond my SPL days, but still like loud, clean audio. My commute is on poorly maintained (noisy) roads and the highway. System must play through the noise.

Daily driver needs an upgrade. It will get a fair amount of sound deadener, some mass loaded vinyl, and some foam. Not complete coverage as this is a budget install. But enough to make a difference.

So back to the question... Clearly a 6x8 has more cone area for moving air mass. But some searching online would seem to indicate that the oval shape of the cone may cause distortion during extreme excursion (very low frequencies) due to the circular voice coil and former being attached to an oval mass in an oval surround. In other words, the load on the motor is out of round. At least that's the working theory I've seen pop up online. I'm not educated enough to form a hypothesis.

How bad would the distortion be? Muddy bass? Actual, recognizable distortion?

Drop-in 6x8s would certainly be easier, but I would have no issue making sufficiently rigid adapter plates if a 6.5" component set would be better from the standpoint of sound.

This is an issue because my sub setup will be a single sealed IDQ8 under one of the front seats. So if 6x8s will play low enough and clean enough to allow me to crossover the 8" lower than anticipated, that would be fantastic. On the other hand, if 6.5's will play appreciably cleaner, insofar as it would actually be noticeable, then that route is preferred.

Comments welcome.