Hello,
When I demod I found the PV1 to be a big improvement over the KEF sub especially for music. Having said that I found the Tannoy TS300 to be just as good, (maybe even a little tighter), which is significantly cheaper. I think there are probably other even better subs out there for a lot less than the PV1 as well. Tannoy themselves have just updated their style subs I believe.
I guess when choosing which to demo you'll also need to look at how high up the frequency range the sub goes, as I've noticed some specifying their upper limit at 100HZ which may not be quite enough to overlap successfully with the satellites.
Hope this helps.