DAC+ DSP channels


I'm trying to build the following setup using DAC+ DSP and RPI4:
 - use DAC+ DSP TosLink IN to connect to soundcard out
 - use internal DAC/RCA OUT to connect subwoofers
 - add AES-EBU interface to connect to main monitors
 - use DSP for IIR filters to implement active crossover
 - nice to have: have an option to switch input from Toslink IN to RPI4-i2s so that it can be used as streaming player
 - nice to have: all outputs should be analog/digital
 - in DSP (ideal case) i would mix two incoming signals (rpi4 and toslink in), use IIR for peq/filters and output two stereo pairs (sub / mains).

I have noticed that there are: 1x I2S IN and 2x I2S OUT channels on the 16-pin header. I would like to connect one or both of those I2S outputs to I2S->AES-EBU board.
I understand - and expect - that I will have to work with SigmaStudio, write custom DSP an be able to debug hardware/software. That's the fun part of this project, i guess.

 - does it generally sound possible with that hardware?
 - can you provide reference for DSP input/output channels, how they map to:
    - TosLink IN
    - RPI I2S IN
    - 16pin header I2S IN
    - TosLink OUT
    - 16pin header I2S OUT 1, 2
    - 8pin "eDAC" header
    - internal DAC connected to RCA OUT
 - is it possible to connect one i2s out bus to two devices at the same time (ie. have internal burr-brown DAC connected to RCA OUT and at the same time duplicate this signal via I2S-to-AES/EBU connected to eDAC)?
 - is it possible to have alternative, analog input? I guess in that case I need to provide i2c-slave device and feed signal to I2C DATA IN on 16-pin header?

Thanks a lot for your help!

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