While I don't like 18 awg power cords, I agree that the amp is the only one that needs immediate high current.
What size power cable?
How “big” does a power cable need to be for upstream components? I get that amps can benefit from a large cable to carry plenty of current, e.g. 8-11 awg. Although my preamp and phono preamp have robust power supplies, they can’t possibly benefit from such capacity, can they? Extreme overkill is the norm in hi end, but is there any benefit to more than 12 or 14 awg on a preamp or phono preamp? Much less digital components.