Receiver / CD player DAC

Can anyone explain why I am noticing such a big difference in subwoofer output when connecting my CD and DVD player into my receiver using the analog input as compared to the digital input. When using the DAC in the receiver, the subwoofer is hardly working but to my ears the sound is more accurate. The midrange and highs seem to be less 'digital like' and easier to listen too for longer periods. When I connect them to the analog inputs leaving the subwoofer at the same level, the bass is much more heavy. When I was comparing them for the first time I was expecting subtle differences but nothing this extreme.

Receiver - NAD T754
CD changer - Marantz CC4001
DVD - OPPO 980H