So wait a minute.... this is a 300M with the stock Infinity system. You said its rocks yet you still swapped out the factory speakers with the 6012i's, is that correct? Now with the 6012's she thinks its too bright. Why would you put the lousy Titan speakers in it if the factory Infinity system sounded good to begin with? The Titan speakers make better paper weights than they do speakers.
That is the biggest problem with Infinity speakers is the tweeters are hash and they just dont sound good unless they are installed with that in mind. If you can get at the wiring to the tweeters you could install an L-Pad and adjust the level and turn them down a bit. The problem is that this will turn the level down but the tweeters will always be harsh.
You can see an example of one here.
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=260-264
If you turn the treble setting down on the radio does that seem to help it?