The following steps will allow you to connect an outboard effects processor through the subgroups when all Aux sends are being used:1. All channels you wish to send to this processor must be routed to 1-2 with the button next to the fader. MAIN should also be engaged. 
2. Bring the SUB 1 fader to the "zero" position. 
3. Verify that the LEFT and RIGHT buttons next to the SUB 1 fader are NOT engaged. 
4. Connect a cable from SUBGROUP OUT 1 to the input of the effects processor. 
5. Connect the left and right outputs of the processor to the LEFT and RIGHT LINE INPUTS of a stereo channel (27/28 for this example) 
6. Bring the stereo channel fader to the "zero" position. 
7. Engage the MAIN button on channel 27/28. 
8. Verify the 1-2 button on channel 27/28 is NOT engaged, otherwise a feedback loop will occur. 

The overall input volume to the processor will be controlled via the SUB 1 fader. The overall output of effects to the Main Mix will be controlled with the channel 27/28 fader.