Want to add a sub to my setup.

[Generic Asshole]

I'm looking to add a subwoofer to my setup while knowing practically nothing about audio so looking for some advice.

 

I currently have a pair of Edifier R1280T'S, hooked up to a Behringer UMC202HD. I originally had the speakers then brought the audio interface when i needed a mic for my pc. I use this with my speakers connected to it through rca in the back while my headphones and my mic are plugged into the front.

So i'm asking how would i go about adding a subwoofer to this without spending too much money, and a suggestion on which sub would work best here (reasonable price of below £300ish). Thanks

 

Edit: Cable from UMC202 to speakers is https://www.amazon.co.uk/gp/product/B004EBKV1Y/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1