Kenwood Car Stereo KDC-135 balanced Left and Right...

Hello,

I installed a Kenwood KDC-135 CD Player in my Echo. I have noticed
some strange problem with the sound, specifically the bass response.
When the balance is turned all the way to the right or left, I hear a
good bass response from the speakers. When the speakers are balanced
equally, the speakers sound very thin with no bass. I've checked all
the settings this stereo has for the type of speakers I installed (6X9
JVC 5 way). The speakers supposed to provide 20 watts per channel. Is
there some type of wiring that I might have misconfigured?