I think your Promac AVR is not a true DVD ready receiver, meaning it is only a stereo amplifier and not a 5 channel amplifier. When you connect the fronts (left and right) out from your Nextbase player, the signal will be amplifed by your Promac AVR and distribute the stereo sound to the center and surround speaker outputs. However, since hindi naman talaga DVD ready yung Promac, even if the center and the surround outputs of your player is connected to the input of Promac, no center and surround sound will be reproduced.
I suggest that if you can still return your Promac, try to do so and replace it with Magnum AMP5121 or Sakura AV-387A.