Hi PKS-10,
Forgive me if I say something you've already tried, but this output on the back of your unit performs one of two functions: either full range frequencies for the rear (normal music output in other words), or subwoofer output, where only low frequencies are output. In either case you need to connect this phono (RCA) output to a suitable external amplifier. If you are going to use a subwoofer connected to your external amplifier, then make sure that the output is set to 'SUB-OUT'. You do this by pressing and holding down the select button until the settings menu appears, and then repeatedly pressing select (briefly) until SUB/REAR is displayed - choose 'SUB-OUT' by rotating the volume control until this appears on the display, then press and hold the select button again until the display returns to normal.
The only other thing I'd then check is that you have some level set for the subwoofer. Do this by pressing select repeatedly (where you will see the sound settings like BAL, FAD etc) until 'SUB' appears and then turn the volume control to set the level you want for the subwoofer. Unless you have some level set here, you will not hear anything regardless of where you set the overall volume level.
Hope this makes sense and is of some help.
