Reply to Problems with the speaker - mixer connection

Hello again!

Now I am having a bit problem with connecting the speakers to the mixer. I am using a little bit older passive speakers, but they are still superb. Now, I was trying to get things going with the speakers, so I pluged them to the amplifier, and connected the amplifier to the Master RCA Output on my Denon DN-X1600 mixer. Now connection between the amplifier and the speakers was good, as I could hear it, as well as the connection between my mixer and Traktor software (I monitored that via headphones). I have my mixer on MIDI mode.

The problem obviously lies between the mixer and the amplifier. Is there something wrong with the mixer settings? What can I do here?

Thanks so muck for help!