My knees are not super either, skied the raichles, they did not give issues, my langes based on the lc11 neither. But i am heavy, so its not easy to stiff for me.
Ski length and stiffness makes a difference. But it depends on skistyle and the way your knees hurt.
I've skied volant powerkarves in 193 and the powerkarve ti in 173. I went to the shorter ones thinking they would give a lesser torsional force on my knees. What i forgot is that the opposite also applies. Meaning longer ski less force needed to make it flex. The same applies to a softer flex. However! this works on medium to higher speeds. At lower speeds your are the one doing much of the heavy work and then length/weight comes into play.
For me the sweetspot is around 180cm (95kg-105kg/180cm) and with my weight/speed i do not want a to soft ski. So i got a couple of Volkl Grizzlt 177, in 2,5 weeks i will now if that gamble is right, still got the volants!
However what i learned...
-Wear a ski/sport-brace! Ask a sport/med which one you need, i am using these http://www.muellersportsmed.com/Hg80_Knee_Hinged.htm
and you should wear 2 in order to get similair feedback on both legs. The ones i use give support without noticable restrictions. This is preventing....
-Sharpen your skis perfect, the better the edge hold the less force you have to apply on hard surfaces.
-Wax/polish them as if you want to win a race. Especially at slower speeds this makes a real difference.
-Use ZardozNotwax if the temperatures are high and you get sticky snow.
-Keep your bindings settings a bit lower as recomended.