check that your fader isn't set all the way to the rear

In AC/Home mode, the player's fader is set to the center so that you get full volume from both the front and the rear outputs. So that's not it.

At this point we should work on ruling out the computer's sound card. Try using that RCA-to-headphone cable to plug the empeg directly into your PC speakers, bypassing the PC, and see if it sounds OK. If not, try using that same cable to connect a CD player to the PC speakers. Also try connecting the CD player to the computer's sound card.