I do have a 3.5 to RCA laying around, but I was thinking optical out of Ps3/4 to optical in on the mixamp then out of mixamp and into the receiver. I have no idea how to actually make that all work when my receiver currently receives the audio and video for both the ps3 and ps4 via hdmi input (separate hdmi inputs for each console).
I'm also wondering how big of a pita it would be to switch the mixamp from ps3 to ps4 in this scenario, which I do often.
I'm wondering if it worked would the sound quality of the onkyo be better than getting something like an e11 and running the mixamp into that. If not, maybe the ease of the set up would be worth the extra $ for a little portable amp.
Audio is complicated!