I’m using the same wire for the supply connections as used for signal. I always think of power supply lines as signal lines, they just happen to be carrying large signal currents as opposed to smaller ones. The wire is low oxygen, silver plated multi-strand.
The positive, negative and earth runs of the power stage of the power amp use separate wires. The two proceeding stages of the power amps use their own wires. The pre-amp has its own supply wires and there are separate wires for left and right channels.
So eighteen wires in total, all of equal length and with each pairing of wires being for left and right channels. This ensures phase is kept consistent and keeps each channel a mirror image of the other.