Kef LS50 and foam port plugs

Do you use the foam port plugs?

Matter of personal preference.

How does it change the sound? 

Acts to limit the bass. Good if you need to put them close to a wall, or using them for Home Theater use.

It also increases the dynamic range, because cone excursion below port resonance is much less.

And can you take them out if you don't like what they are doing?

Yes, of course.

I have  the plugs in.  If I did not use a sub with them plugs would come out to extend the bass.