The crossover had a constant impedence of 8 ohms. Not sure if Kef tried the idea around the same time. There's a posting by the son of YHF speakers on PFM. He gives a few details.
Adrian Orlowski of Hi Fi Answers was quite a fan of them.
I think Kef's implementation was known as the Kube. I could just be talking bollocks here though. Anyway, the idea was for the amp to see a constant (more or less) load.

