As we normally tell people, without actually hearing then yourself and deciding which "you" like the sound of best, (individual taste in sound is very personal) the differences between the two speakers you've mentioned make it even more so because you're comparing Bi-poles to Mono-poles and they tend to have very different sound qualities, which creates considerably varied opinions on who likes the sound and who doesn't.
In general, while bi-poles can image very well, they are far more placement dependant and that imaging can be more diffuse, which some people just don't like.
Plus Definitive and Kef are not extremely neutral by design, they both have a distinct sound that they are trying to achieve, so you will really have to attempt to hear them without relying on a coin toss, as to whether you like them or not.
All of that said, I personally am a DefTech fan and still own an original pair of BP10's from the early 90's and have owned BP10b's also, and being bi-poles they will need a little more room to breathe and for placement tweaking in order for them to reach their full potential, but... the BP10 is very musical and well suited for HT also, it will play cleanly down between 35-30hz. It's actually one of my favorite DefTech speakers and is their longest single running model.
I just don't know the Kef's well enough to give you any insight with them.