S4 soundcard and cue issues

O.k first of all apologies as by searching on Google I know these problems have been covered on this forum separately before, but the advice given on each problem does not solve mine.

I have just purchased an S4 secondhand and have set it up today to test and play, I have no doubt the unit itself works fine. If running out from the s4's soundcard to my speakers, I get occasional popping and loss of sound. After reading the forum for solutions the best suggestion that worked was to run through a separate soundcard.

So now everything sounds fine, no problems with sound and the mixer is great, apart from when I go to use the headphone cue which now does not operate at all. This worked before when running straight out from the s4.

Any suggestions?