Dear bartokfan: +++++ " This is the way loudspeakers behave. Loudspeakers are not constant power devices. Loudspeakers must have a constant drive voltage to provide a constant acoustic output with changing frequency. As demonstrated by the simple mathematical relationships shown above, the only way to achieve constant voltage drive with a changing load impedance is by having very low amplifier impedance. " +++++
This is what you can read on the link you posted, as you can " see " this subject is critical.
Now, the tube amplifier technology has many trouble to handle in the right way all those speaker needs, so ( if you want to go with this technology ) you have to make big trade-offs with it.
Of course that you could choose for SS technology that could mate in a better way your speakers needs and your music sound reproduction priorities.
Regards and enjoy the music.
raul.