Curious, then why don’t speaker manufacturers do this? Wouldn’t they want their gear to sound the best possible?
Honestly, I think they are pushing to go 4 ohms and lower to give the appearance of the speaker being easy to drive because generally speaking, as impedance goes down, sensitivity (as a measurement) goes up. The problem is sensitivity isn’t the same thing as efficiency, and with tube amps in particular efficiency is a much more usable specification/measurement! Beyond that, my opinion is they often haven’t made the connection between distortion on the spec sheet vs. what they hear. That increased distortion is audible as increased brightness and harshness.