Audio Systems: OEM stereo, Adding better sound, chevy tahoe, channel amplifier

Question
Hi, I have a 03 chevy tahoe with OEM 6 CD stereo i have added a amp and 12" cerwin vega sub-woofers that i got from my bro, but they dont sound that great(my bro had them in his 03 navigator oem/stereo) is there anything i can add (with not spending a lot of $$) to make sound better...?? Thx.  

Answer
are you using the correct hi /low adapter ?
you have to use one made for the vehicle, if you want good quality audio  

http://www.oemautosound.com/c-135-gm-chime-retention.aspx

also you need a small 4 channel amplifier for the inside speakers