Main issue with multi-channel signals is the signals can be transferred via HDMI, USB, and analog. You can't split off the front channels digitally via HDMI, so the only way to split the channels is via analog. USB is limited to 8 channels and connecting via USB from my Raspberry Pi server could be problematic.
If I only had a 2-channel system, it would be simpler. I am connecting the Raspberry Pi server to DAC via I2S, which I could insert a DSP in between. So the Toslink/Coax connections to my DAC on my input board become useless, but additional digital inputs to the DSP would replace the ones not being used to the DAC. I also have an analog input from my turntable which would need to be connected to the DSP. This requires an ADC to get it into the DSP. When you add up all the IO into the DSP, it ends up costing quite a bit.
In the end I decided to go with this train:
Raspberry Pi via I2S > DAC > Preamp > amp > Speakers (with passive crossover)
If I am not satisfied with the passive crossover I can always add a miniDSP 2x4HD or greater if I want more sub control.
Raspberry Pi via I2S > DAC > Preamp > DSP > 2 stereo amps > Speakers (2 way active)
I can also use the other inputs on the preamp for phono and HT (Front Left and Right):
Phono or HT > Preamp > amp > Speakers (with passive crossover)
or
Phono or HT > Preamp > DSP > 2 stereo amps > Speakers (2 way active)