6.5 bar is definately too low - it should be 8.5 to 9 bar. Also what is the temp of the group at extraction - if not at about 93C that can cause the flavour to deteriorate..... You might need a cooling flush to remove the overhot water etc... or even a flush to heat the group (dont know the CK myself)
2-3 mm in the basket can make quite a difference. Is there a slight imprint of the showerscreen in the puck after extraction? Reduced depth of coffee- easier water flow- faster extraction = poor taste!
I find my standard (14g) La Cimbali basket takes about 19g when correctly filled and tamped so there is just the slightest imprint on the puck.