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. 
Well SET stands for single ended triode, no? Unless it's used as an acronym.. "(an)" ess, ee, tee...phonetically. But then that would still be wrong since the first letter of each word is only simply spoken but not as a "word". Weird
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.