I think I see what the problem is - you need to use your ES-8 for your master audio output from your iOS device. You cannot use both the headphone jack and the ES-8 at the same time because iOS does not support aggregate devices.
When you plug in the ES-8, it becomes the only way to monitor sound on your iPad. Ouputs 1&2 of the ES-8 are your master audio outputs.
If I had my modular set up with my iPad controlling the ES-8, I would reserve outputs 1-2 and send those to my audio interface on my computer, where I can monitor the sound there.
When we update Audulus to include support for aggregate audio devices, you will be able to use Audulus on a Mac in standalone mode without needing to use outputs 1-2 to send audio out to your interface. You will simply be able to select your headphone out to monitor from within Audulus using, say, DAC outputs 17-18 (since ES-8 has 16 total outputs with the ES-3 expander).
I don’t know when this problem will be addressed on iOS, but it’s not something we can do anything about because there’s no audio preferances pane where you can even create an aggregate device on iOS.
In short, you can’t do what you want to do - both use ES-8 with Audulus to send CVs and then process audio downstream with your DAW and monitor it on your iPad without reserving outputs on your ES-8 to send to an external mixer. I would not recommend plugging the output of the ES-8 into any stereo without some kind of attenuator in between, as it puts out 10x the power that a stereo is usually expecting to see, and would likely fry it.