Hi Michael

> 1. In direct comparison with the A-105 SSM Filter, the output level is
> about 20 dB lower. I set the input levels of both filters to a level,
> where the signal is at the maximum level without any distortion (which
> is about half way up on the A-105 and somewhere slightly below the
> maximum input level on the A-121)

The A-105 has incredible power in comparison to all other A-100 filters,
it is my absolute favourite A-100 filter. The A-121 could be driven with
more input signal power (you might reduce the inputresistor after the
gain potentiometer).

> 2. At low(est) fc settings, the highpass sounds like it is filtering
> high frequency(!) components from the signal. I don't assume, that
> this is normal behaviour, or is it

Yes. This is mentioned already in the Datasheet of the Curtis Chip.
There is a modification which traps into more inconvenient resonance
behaviour.
I tried it once, but took it back, because I did not like the resonance
thing.

> Furthermore: With fc at maximum, I still hear the signal. Is there a
> way to increase the range, so that at the highest fc setting, the
> signal is inaudible

Might be trimmable. Don't have the schemoes here. Maybe you have to
weigh up against how low the filter goes on the low end (you might run
into the samething with the lowpass filter setting on the low end).

> 3. The notch-filter seems to behave simliarly to the highpass, in that
> it seems to filter high frequencies, when fc is set low.

Yes. Know behaviour as above

> Are these three points common problems, that are explained in the way
> the module is built, or is my A-121 simply defective

Your module in general works as it can and should do. Topic 2 might be
a question of trimming (I cannot judge from here).

Florian