Speakers don't produce hissing noises--not even cheap ones. Hissing
noises come from noisy electronics. However, if you're talking about
*scratchiness* then you might have a roasted voice coil (or two) in
your 6x9's. A pretty simple way to tell if you have a driver with a
really roasted voice coil is to gently move the code back and forth by
hand, making sure that you apply even pressure to both sides of the
cone--tolerances are pretty tight, so if you push on just one side or
push unevenly, you'll wind up rocking the cone out of alignment and
scraping your voice coil up against the walls of the gap (bad). When
you move the code back and forth, you should not feel any scraping--the
cone should move freely.

As for component speakers being better than 6x9's, well, yeah, they
typically are, but not for the reasons you've cited here--it has more
to do with the amount of money manufacturers put into making the
speakers. Six-by-nines are specialty speakers that only fit into
relatively few vehicles--so you're not going to get audiophile quality
in that format....although my understanding is that JL Audio makes some
pretty good 6x9's:

http://mobile.jlaudio.com/products_components.php?comp_id=129
http://mobile.jlaudio.com/products_components.php?comp_id=128

I know them by reputation only--I've never listened to them.

Speakers mounted in the rear parcel shelf should suffice, and will
actually have a higher probability of producing "nice, rounded bassy
tones" than speakers mounted in the side panels--the latter aren't
typically very rigid, nor do they have a lot of air behind them, which
6x9's tend to want.

-dan