The Denon 1802 has a subwoofer "pre out" that you should use to connect to the EP125 (via one RCA patch cord/interconnect). That's all you need to connect. There will be the proper RCA connection(s) on the EP125. Leave your other speakers connected to your receiver. The external input on the back of the receiver is for a separate discrete 5.1 program input - such as another DVD player that has a DD and/or a DTS encoder built in. That way you could use the decoder of that DVD and not the one built into the Denon.
Do not loop back a connection from the subwoofer to the receiver's ext. input (BAD!)

Hope this helps.