Subwoofer for Kef LS50 Wireless II?

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.

I absolutely agree with @mapman, ISOLATE the sub from the room with an acoustic platform or springs. If you can't afford a very good isolation platform like Townsend, Auralex Subdude is decent. The aluminum version of the Nobsound springs will work for $35. Start with the lowest crossover point on the low pass filter, 30Hz. or lower? If all of this does not help, stuff the ports with cotton or fiber material. This should reduce the boom. I think the BEST solution is isolation!

Hello, I own two Rel T9’s. With the Speakon connections they sound really good. The store in the Chicagoland area has both of the new KEF subwoofers.
https://holmaudio.com/
 They both claim to play down to 11hz at -3bd, but the bigger one which is the KF92 plays at 110 db and the little one which is the KC62 plays at 105 db. Since my system has the sub outs not being used and the KEF has a wireless kit I thought it might be good to integrate it to play from 11hz to 30hz which my RELs are missing. 
I don’t know if this will work but the store listed above let’s me try before I buy. I tried to borrow the bigger one last week but it was out on loan. These subs are so popular now. It wasn’t a total waste since I borrowed a power cord to demo. 11hz is really low. Does music even go down that far?  I hope this gave you some ideas to try since I am going through the same thing.

thanks for the feedback
i think ive been convinced to stay the course with this sub a while longer
i just ordered a subddude
also just heard a very good bass track , Tidal MQA version of Cardi B's Bodak Yellow, this is the first track that this sub has impressed me with, interestingly enough this particularly track it does not rattle my house

the KC62 is a killer sub considering size and $$. Its worth a look.

A new review by Herb and Stereophile

https://www.stereophile.com/content/gramophone-dreams-49-kef-kc62-subwoofer