The earthing for a mains powered Hi-Fi system is pretty straight forward, so long as you keep the signal currents away from the heavy currents placed on the power supplies and don’t introduce any loops etc. The earthing for the battery driven system is even simpler.
Mix the two together, add in the factors of common earths for the jack input and headphone output plus the sub-woofer circuit which will be mains powered regardless of whether of the main amps run off mains or battery – and all hell breaks lose.
I’m getting there but I’d be lying if I said it wasn’t a struggle.
Note that the above drawing was taken before completion and contains errors – some quite big ones if you look closely!