This is all the more reason to go and listen to the MB Quarts, and at least one other brand. Music preference is a VERY personal preference, and different people will like very different things. (NEVER buy speakers without listening to them first)
For example: (I have this fresh in my memory since I just went through a listening session with the top 10 speaker brands)
The comments that DJ made about the high-end of the Infinity's is similar to a comment that I might make about the MB-Quarts. The highs are quite fatiguing for an adult listener. (Unless you buy the set that costs $1600.00, those I really like.)
On the other hand, I didn't think the Infinity's were fatiguing, but there were other things I didn't like about them.
The JL speakers sound just like every other low-end speaker out there. Instruments do not sound like real instruments, rather they sound like cheap recordings of the real instruments. Most kids probably would be happy with them for their boom&sizzle factor, but if you're after something that sounds better than the stock setup, then you're looking at a deeper investment.
Note: Notice I did not whore out my own brand.
One other thing... If Sound Quality truly is your goal, don't have two sets of speakers in front of you in two different locations producing the same frequencies. (For example: two tweeters, and two midbass drivers, dash, a-pillars, doors, and kicks on each side) This will smear the image, and present you with a confusing sound stage.