Subwoofer for Kef LS50 Wireless II?

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

I reading this thread with great interest.  I have an SVS SB2000 Pro and was wondering about how a set of the wireless ll’s would would sound with it.

JD