Normally what the effects returns are utilized for is to input the outs of effects units (duh...what's in a name...). This would be for send-type effects, not insert effects. So you have the output of aux 1 going to a reverb unit, and aux 2 going to a delay unit, and then the outputs of those units come back to the mixer at returns 1 & 2 respectively. Those returns are typically routed straight to the main buss, or in the case of a multiple buss mixer (like the 1508 which has those 4 groups) you can have a little more control as to what you do with those returns and where they go, but in your case if you used them to input the outs of the other two mixers you would just route the effects returns on the 1508 straight to your main buss. BUT, I need a more detailed picture of control panel of the 1508. BTW, there was only one picture I couldn't open, and that was the one of the G24S. I can view it today tho', but all the pics are 320
x 240 and are way too small for me to see what I need to see.
Did you scale the pictures down to upload them to image shack?
Use the 450 for G24S outputs 1 ~ 8; output to the 1508 via the MON OUT jacks. The level is controlled by the MONITOR knob under the group 2 LED level meter.
Use the Yamaha for G24S outputs 9 ~ 16; output to the 1508 via the REC OUT jacks. The level is controlled by the ST fader.
Use the 1508 for G24S outputs 17 ~ 24; again, I will need a more detailed larger picture of the master section controls on the 1508 before I can tell you for certain what to do as far as connecting the 450 and the 12/4.
I can PM you my email address if that would be easier as far as sending me the picture.