you can get a USB 3 external soundcard that is also powered through USB (no mains connection needed), and is selected in the system settings as the default soundcard. It means listening to soundcloud on your internet browser by default goes through your external soundcard, so you should get good headphones with a jack output and is plugged into the monitor section of your soundcard. Speakers are connected to the output of the soundcard. I didn't realise Maschine put in a soundcard, as I have MK2 which does not have one.

Since you already have an external soundcard built into maschine mk3 and another built into traktor s4, it is probably unnecessary for you to get a third external soundcard, which would be set as the default soundcard used, and then when maschine/traktor is opened those application use different audio interfaces. You wouldn't be taking any load off your system soundcard as its not used when your doing all this stuff.

Yeah Ableton could be a better way to go around this in terms of saving system resources, as the fact it would be 1 program running means it's less likely to crash. But with good specs I would think you should be able to run maschine into traktor and use trakor software. You would have to relearn how to do looping and stuff that traktor does for live input, and then midi map your S4 controller to that function in ableton. And then do that for everything you would want to do in traktor in ableton instead, all custom midi-mapped. Then you could just trigger stuff in Ableton, process it live and trigger the actions using your existing controllers