We need to know what kind of connections you are using.
HDMI
DVI
Component
S-video
or Composite
You seem to be saying that you have a composite cable (red, white, yellow - the yellow carries video, the red and white carry audio - red=right channel, white=left channel), and a component cable (green, blue, red - all 3 for video only). If your set has a component input, connecting the component output of the cable box to the component input of the TV will give you the best picture. If you're set up for HI Def, you'll Have to use the component hookups.
Can you give me the Brand and model # of the cox set top box (for example,I have the Scientific Atlanta Explorer 3250 HD), and the Model # of the Phillips TV. I can look them up on the internet and get an idea of the best way to hook them together.
I'll go look up the Yamaha 750 to see what kind of audio connections you have to make.