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. 
What I wanna know is why It's always referred to as "an" SET amp instead of "a" SET amp? "S" is a consonant! Sheesh :)
I refer to an SET by saying 'ESS EEE TEE'. If mentioned any other way its usually 'single-ended triode'. I hear 'set' far less often.