I've never used Infinity's 6x9s, but I've used their 6.75" Kappas. I hate them. My stock 3.5" dash speakers put out more bass. As far as the midrange and treble is concerned, I'll just say it's tolerable -- definitely not desirable. Overall, they're worthless IMO.
Infinity Uses JBL crap that is dumped down.. if its not the JBL GTI series than don't even bother.. their highs are shrilly as hell even with the 3DB..
They will take punishment but at the end of the day.. spend the money on something better.. Infinity is the unwanted step child.. stealing your corn pops..
Youll need a good baffle and a little customizing but a buddy has a pair for 2 months now.. with the right setup you can make it sound like hella deep, and nice and crisp.. made in the UK no china crap..