I posted this on another section but it answers your question
the 4x6 is a bad speaker, no matter I would convert to a 4 or similar.
This is what I did and the sound difference was very noticeable.
"I am currently running a kenwood
deck (with bluetooth, ipod, etc, etc). I wanted to upgrade my speakers to a quality full range speaker.
First thought was Boston Acoustics, after talking to a few people and getting on some of the audiophile sites; I found that Polk (which sound very much like Bostons) use enough stainless in their speakers and other hardy materials that they earned the marine rating!
Long story short I got a full set of Polks, the wiring adapters for the speakers, 4x6 to round converter plates for the front dash, baffles for the dash and polyfill for the speaker bar.
All in just shy of 160 from Crutchfield.
Plus the speakers arnt white (not that it matters anyways as I'll be using the stock grilles)"