DLM-800 Send & Return Ports with RMX-1000

Hi

I am trying to use the Send and Return ports with an RMX-1000. When I connect the send port on the back from the mixer to the RMX-1000 input I get no input, there is nothing coming through on the RMX Input db scale of green lights.

If I change the cables in the back of the mixer to the Booth port I get a full input on the RMX-1000.

I have tried the same test with a speaker, the Booth port gives a strong sound, the Send port gives a much lower volume. The Return port works fine.

Any ideas please?