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In this case though, what other modules could I add to create something interesting?

It depends what you are interested in! If you click through the modules here you will find many video clips showing what they do. Alternatively maybe try VCV Rack and experiment with some things to find out what you like?

• Can I use the A-138o also as an attenuator before the befaco output?

Yes, but there is not much point, since the Befaco module has an output attenuator.

• Why I need also an attenuator for input? In this system I have only midi as input signal, I'm probably wrong but I want to understand.

For an audio input to the modular you will need an amplifier of some kind, not an attenuator, otherwise the audio will be much too quiet compared to the modular gear. For MIDI input this is not relevant, you just need a MIDI-CV converter.

• On the Digitone or Octatrack you can sequence multiple tracks so what can I use to split my midis into cv?

It depends on the converter, but different MIDI channels or cc messages could be different CV signals.