Good to see the 556 get a namecheck!

How is your bi-amping arranged? Does the 556 handle bass, or the 551? If the 556 isn't handling the bass, I'd suggest you try that.
I did think about getting a pair of Kef Reference speakers, and reviews I read
suggested that the bass was well controlled, but not very extended given the size of the box. I also read that
they're not the easiest speaker to drive, which can lead to flabby bass. You could try one of Meridian's more
muscular power amps, whch might be better able to control the Kefs.

(I upgraded from an Audiolab 8000S to a 556, and found it improved no end the bass
which my Ruark Talismans delivered.)

Hope that helps,
Mark S