OHMs law on S.E.T. Amps

A quandary I’m unable to find an answer on, even asking brick and mortar dealers is: why do SET tube amps output wattage decrease with lower impedance loads (speakers) in comparison to SS amps wattage output staying the same or even increasing with lower impedance loads? I have a rudimentary knowledge of OHMs law, but by no means claim to be an electrical guru. Any explanation would be appreciated. Layman’s terms explanation is just fine for me.