Send/return works like this:
The P-520 is your "send x 4"...you'd patch a signal to its inputs, then the stompers' inputs to its outputs. For the return, the A-520 works the same way...stompboxes go in its inputs from the boxes outs, then the outputs go back into the modular. This gives you either four mono s/rs OR two STEREO s/rs, which these make much easier due to the fact that the i/o levels on the Ladiks are identical across the four input/outputs on those. No potential of fiddly accidents changing the levels! All you need are 3.5mm to 1/4" cables for the connections...the rest is patch and forget. You'll run your stompbox inputs a bit lower than you would with a guitar, also, since these also work at line level. BUT...since they DO work at line level, you can ALSO use these for any sort of send-return scheme. Want to run things to something like an old SPX90? You're good with this.