I tend to only use the mono L output for the main outs, but this is entirely a matter of taste/channel availability.
You don't need any TRS cables, its all TS.
edit: oh wait I see, for the DBX on the kick drum...
well you run TS cables from your 909 to your desk inputs and then if you have an insert socket on your desk you can use a Y-split "insert" cable for the DBX... with TRS end plugged into the insert socket on kick drum channel of the mixer and the two TS ends going to the input and output of your DBX.
