The reason a lot of car audio 12s sound bad is that their magnet is too weak for that size of cone (manufacturers often save money by using the same magnet/vc assembly for a arange of drivers eg 8", 10", 12" and sometimes 15" all with the same magnet. Generally, the larger sizes of cone will be too massive for the magnet, giving a high Qts, which results in the bad sound.

If you can find a car 12" with a decently low Qts (eg .2 to .35) then you probably won't get the sound quality problems you mention. Actual cone size isn't the issue - its the bad mechanical impedence match between a weak magnet/vc and a large cone.

Cheers, John