Hi there. 
I am a bassist.
Let’s say the scenario is a 4-cable method to a real Amp. No amp simulation, no cab simulation or IR will be in the way. Only the real Amp and it’s real cabinet. No PA, no FOH. Nothing. Just the QC and the real amp-cab.
So, somewhere in the signal chain there will be a send/return block identifying the before and the after the real amp. Some effect blocks before that and some other after. Till there everything easy.
PARALLEL PROCESSING
At the early stages of the signal flow (before the real amp) there will be a split frequency crossover, low frequency band (150Hz and below) in one path and high frequency band (150Hz and above) in the other path. So, I can heavily compress the low band only and I can add overdrive / distortion on the high band only. In contrast with the guitar world, this is very useful for bass sound not to loose its bottom end and retain clarity on the high band.
Somewhere in the signal chain these 2 signals must be merged again.
By your experience where you would merge them? before the real preamp or after? It sounds like a silly question but I am a bit confused due to lack of knowledge on parallel routing.
If I merge them before the real preamp will the 2 separate frequency bands translate through the real bass cabinet as I set them with the split crossover? Will I hear separately the clean and heavily compressed low end with the heavily overdriven high end through the real cab? Or some other routing should take place?
Thank you very much my friends.