Low-pass setting

I've observed the in-room response of my KEF Reference 1s using the SMS-1 display, and it comports closely with the KEF specs.  There is a gradual roll-off just above then a sharp drop-off just below 30 Hz.  That leads me to expect KEF's specs for average in-room response for the LS50 may be pretty representative.

6db is what you want to use as the 3db point will give you a hump at the crossover point. If you can highpass (cut the highs to the mains) then 80hz would probably sound better as the “bass” drivers in the Kefs are tiny and will need to travel pretty far to make a lot of output). 
Keep in mind the 6db point is NOT where the specs are but where they measure 6db down in YOUR room. 
60hz is a good safe starting point for most small speakers. 

In my main setup I use a Velodyne SMS-1 bass manager that uses a microphone to provides acoustic room correction for a pair of HGS-15s.  I suppose it might be possible to use a SMS-1 with a KEF KC62.

the tools are not very expensive….. but not nearly as sexy as a new fuse or magic stone…

100-120 is the max unless you want to mess up the stereo image, Very very few systems utilize low frequency EQ and owners w tools to measure in room bass at the listening position. Some of us are blessed.

We usually specify the -3dB point for filter settings.

A lot of audiophiles start off being "purists" of a sort, trying to get the sub to do as little as possible and then end up switching to a higher setting because of the improved clarity and depth.

Even with large speakers I’d recommend 80 Hz or so as the crossover point for sub and speaker. It’s hard to set precisely without measurement or without knowing how fine grained your controls are. You may benefit from a LP filter slightly lower than the HP filter.

If you are using them with a subwoofer, I also suggest plugging these speakers.  This will improve the crossover function as well as increase their dynamic range.

One of the big advantages of using a sub is being able to use EQ on the sub alone, so the less your mains are doing the more control you have over the bass.