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

I certainly would NOT worry about the high level inputs. I've always found the low level inputs to be preferable. Simply more detail. The SVS, while not the best, should be very good with your speakers.

Non sense...On the Rels use high level....Although i havent heard the Tx series ive had some first hand reports,  I wouldnt waste time ...go for their best selling sub in their lineup if you can swing it.. the S510...will step all over this Tx series...im running a stereo pair coming off the Ti series...fantastic

OK, missioncoonery. You must be an expert. Just gave my opinion from MY experience.

How are you going about it?

I have Ls50 metas and a Klipsch sw308 sub in a smaller 12x12 room. Larger rooms are likely more of a challenge in that the sub will have to do more. Listening to my ls50/sub combo as I type. To get the sub set up right I used a sound meter app (decibel) on my iPhone to fill in the lowest octave that was missing in my case mostly below 40hz or so. You need to measure to get it right. I had crossover set way too high by ear. Using a white noise streamed internet channel as the source I Turned up sub level then adjusted crossover point and tweaked those two until low end was leveled with the rest. Then tweaked phase with some good percussion recordings for best dynamics. Could not have done it right by ear alone. Sounds awesome. Sub needs to be able to go down to 20hz or so in your room to fill in the bass completely. Check the specs. The new KEF sub is designed to go with the ls50s. I’d look at that first.