Hi, you have several options here.
A) If you want to connect via the headphone out (to send the same signal to all headphones), then you can connect this with a stereo 6.3 plug (not with a usual mono 6.3 plug, otherwise you would have a mono signal instead of stereo) --> item number 3488031.
B) If you want to have 4 extra headphone outputs in addition to your headphone output, you can connect the microamp via output 1 and 2 with a Y-cable --> item number 6060778.
C) If you want to have a separate headphone mix to the Microamp (e.g. via a DAW or Focusrite Control software), then you can do exactly the same as in option B, simply via outputs 3 and 4 (so you can connect monitor speakers via outputs 1 and 2).
I don't know how it works with the Focusrite. I have connected it to the Horizon Box from UPC via Cinch (jack plug). It works fine, but there is always interference with the headphone cables (I have already changed the cables several times).
Hello Anonymous, normal 6.3 male - male audio connection cable. If only 3.5 male - male audio connection cable is available, you should use 3.5 female - 6.3 male adapter. Have fun.