Thank you for the responses, the cd player is connected via rca cables as it does not have optical outs. As for using a power amp, that's the route I'm gearing towards (once I get the funds for it) I'm thinking Marantz or a NAD power amp. Any suggestions?
If your CD player has the red/white for L/R, then it is feeding analog signal to the Yammy. This means the CD player has its own DAC.
Have you tried:
New set of RCA cables
Plug a different source into the same CD input to test the input
Plug the CD player into a different input
If your sound is good on everything else, then it must be either a cable, the CD player, or the inputs on the Yammy. All of those are easily tested.
Also, be sure that you don't have some DSP mode turned on for that CD input, that could do it too.
If you do the things I suggested, it should be pretty dang obvious where the problem lies. If not then report back your findings and we can point you in the right direction.
My $ is on the Sony CD player due to my personal distaste from Sony. Also, highly likely to have a bad cable.
As far as and external amp....I doubt you will NEED it with those speakers.