quality difference between headphones and stereo/digital out

There appears to be a noticeable difference between the quality of sound from the headphones vs. the stereo and digital outputs. When i listen to the mpc 2500 through headphones the quality is good, but when i play sound through the stereo or digital outs through my speakers the quality seems degraded. Also, when i play the same samples saved on my desktop the quality seems fine and consistent with the quality in my headphones. There seems to be something going on with the stereo and digital inputs and outputs that lowers sample quality slightly. I made sure that my samples were converted to 44.1 16 bit before loading them into mpc. Im hoping to find a solution that can help maintain the quality that is heard through the headphones on mpc and the desktop of my computer.

I think it's hard to compare like you do, especially when being loud on closed headphones - it masks some spectrum giving you false response on your mix. If you want to compare plug in jack from headphones output to speakers.