I think you may have misunderstood me Vintage..
What I was getting at is that Ableton is just seeing a 4-out soundcard. And I doubt that those 4 channels are routed to the 4 decks separately -surely it should be an 8-out soundcard if that was the case?
I think that the SX in this case is simply being used as a midi controller for the decks in Serato (depite being a mixer itself) and that the soundcard is simply presenting itself as a master out + headphone out – even if Ableton doesn’t support this.
Meaning that if you sent any audio to channels 3-4 it would just come out of the headphone output. I don’t know why this is the case (and indeed only the OP can confirm anything when he/she tries it out) but I doubt that the output channels correspond to the channels on the SX..
bob6397