Subwoofer for Kef LS50 Wireless II?

What kind of problems are. you having? Is the sub boomy or muddy?

Boomy would probably be do to the subs location in relation to the listening position, muddy would probably be do to improper crossover setup. 

I'm assuming you only have one sub.  Have you placed it in your room in the most optimal location? (google sub location crawl method)

Set the crossover on the sub at 45 Hz and the gain to 75% as a starting point. Plug the port on the KEF's

There is a document on the KEF website pertaining to integrating the LS50 to a sub.

https://us.kef.com/pub/media/wysiwyg/ls50/LS50WII-Subwoofer-Setup.pdf

What is your crossover point? If it's at 40Hz there are very few notes produced by conventional instruments lower than that. The low "E" on a bass is 41.203 Hz which is already above that threshold. A grand piano has a few notes that are lower but they are seldom used, so depending on what you are listening to, you may only be hearing one or two notes being reproduce by that sub. Distortion maybe an indicator that this particular sub is too small for your application. Not sure what kind of nuance you are thinking you should be hearing. I would say that there is not a lot of nuance below 60Hz

@jetter welcome to the dark side of thinking out side of the audiophile "sub bass" box. Have you ever considered using an external crossover? You may find the results quite dramatic.