It seems like we are talking about two different things and possibly speaking on different frequencies (pun intended - that's as good as I get!)
If I have a tube amp that has a designed output impedance of 8 ohms (not the tube's output, but the speaker out jack on the back of the amp). The ideal speaker load is 8 ohms. Yes, there is a large transformer to couple the output of the tubes to the speaker connection. But that's beside the point. The amp's output on the back was designed to see an 8 ohm impedance speaker. The original question was whether it would be better to connect a load of less than or greater than 8 ohms. I built many amps myself (not designed) and worked for a few years as an assembler for an amp maker (who shared lots of information - I learned a lot). We would always install a shorting jack for the speaker out because it was better than providing an infinite load if a speaker wasn't connected. It was still bad not to connect a speaker, but it was better if it was shorted than open. So, for tubes, I'd rather provide a lower speaker load than stated than a higher - knowing that the load intended for the transformer (however large) would be the optimum load that would work best to couple with the tube's output impedance.
I have a Boss Katana amp (fun, relatively cheap, and sounds pretty good) that is solid state. The Boss states a minimum of 8 ohms in the manual - and I'm assuming that they don't want you to go lower for a reason. So, it appears that it is the opposite for solid state.
I hate to quote forums, but here is just one page where every single answer supports what I just said: https://www.quora.com/Can-you-use-8-ohm-speakers-on-a-4-ohm-stereo-system
Thanks for making me think!
Have a great day everyone!