A 6x9 or 5x7 speaker will never sound as good as a round speaker. The oval shape just isn't good for use in quality sound systems, IMO.
Go with a 6" mid speaker or get some component speakers for the rear that comes with the mid, tweeter, and crossover. You'll be happier with a setup like that and the sound will be crystal clear depending on the rest of your system and the type of speakers you buy. I like Sony's myself, but that is just because I can get them cheap through my mother's work. My preferred choice if cash was permitting would be MB Quart. But they are really expensive.
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