> Is the 106-6 basically a more improved version of the 121 Or are
> they completely different

The basic principles are different. The A-121 is a state variable filter (an
analog realized oszillation differential equation below self resonance), the
A-106-6 is basically the usual 4 stage CEM-OTA based 24dB low pass with the
additional XP circuit. This circuit derives from the 4 stages of the 24 dB
low pass the other filter types by addition/subtraction. One can get a lot
of different filters by using the 6, 12, 18 and 24 dB output of a filter and
add/subtract the four signals with certain levels.

Best wishes
Dieter Doepfer