How profoundly irritating. It looks like that resembles what I saw at the upper end of my LPF knob. First 4 clicks down did nothing, LPF remained at 140Hz, rather than stepping down through 136/132/128/124Hz. Then 5th click gave 120Hz as expected, then smoothly down at 4Hz per click.
But it appears your fifth click is still somewhere lower than it should be, then your sixth click is probably correct.
So you've got this dodgy KEF knob input which is only dodgy at its extremes (maybe they thought they'd get away with it), but that then collides with what seems to be an unusually loud output from the PowerNode. Either one would be okay on its own, but together
Sticking a 9-10dB RCA audio attentuator in your link might solve your problems. (Although you might then enter the world of reluctant wake-up). I guess just 3dB would do - you only need to get past that 5th/6th click. But complain to the Powernode people. It's pretty poor to make a device with no output level control for the sub, and if they're not going to have a control it really shouldn't be outputting at the equivalent of a receiver at "+10dB" trim.
Oh, and complain to KEF too. Their knobs should work. (Are they really analogue pots? Are they that much cheaper than a nice digital encoded knob?)