Buy a pair of competent subs that can go clean and flat as high as 100-120 Hz, because LS50’s response starts to weaken at these frequencues especially if you have them far from the back walls. If possible, cross them over at 100-120 to further relax them and improve the mids. Keep the subs close to the ls50 and you will enjoy.
You can leave them full range as well just dial the sub volume adequatelly.
I enjoy them with a single b&w asw608 sub and the bass is adequate at the listwning position and very clean and articulate.