I use my d8b for a live musical production on stage, so I needed subgroups that can be routed to aux and effects etc. You can't do that with channel groups.
The solution is simple: I put an opt8 into the aux I/o slot and routed it's output back to the input.
This works fine, but I wanted the optical output available for multitrack recording too.
So I modified the opt8 so that the optical input is disabled, but the loop is done internally by a simple piece of wire
This mod is very simple, if anyone is interested post here, I could make a how-to soon.