I made it and it works perfectly. I've made pach that use the stereo loop returns wich goes on the B path at line level and put a gain block on the end of each path for volume control ( i could have use the mixer volume too ). I've put an distortion, eq, comp, delay, reverb onpath A. The eq level controls the quitar level in my head phones, and the path A gain
control ( compensate ) the level in my amp. I assigned the iphone to a switch to mute it at will.
This set up is mostly for portable practice kit with a iphone and requires a stereo a 1/8 trs+ to 2x1/4 ts+ and a 1/4 trs- to 2x1/4 ts+ adaptor. Don't forget to switch loop in/out to line. Since there is no headphone amp the sound is really nice.