The thing is I can't possibly figure out a way to connect the Sub to my audio Interface (Focusrite Scarlett 2i2), so I asked the customer service and they said I could hook the Sub up to my audio Interface with 2x TRS male -> RCA female Adapters and then add a Y cable to hook up my Sub
Does that work? Because that would mean my speakers and Sub would be hooked up seperately to my Interface, thus needing a crossover?
Almost all powered subs have a low pass crossover which is what the controls adjust.
Most subs don't have a high pass crossover so it's the same as splitting the output that goes to the full range speakers and connecting both them and the sub. It's better if the sub has stereo inputs. Using a Y splitter to combine left and right doesn't work all the time. The right way to do it is http/www.epanorama.net/circuits/linesum.html