[Niteshade]

Speakers are rated with a nominal impedance. Let's say it's 8 ohms. Too bad the speaker didn't stay that impedance within it's frequency range. More than likely the speaker's impedance/frequency graph looks like an old, winding country road.  There may be some frequencies that have a 12 ohm impedance and others with a 4 or 6 ohm dip.

What can trouble a tube amp is the low impedance dips. Many people have found changing output taps works for them. If something does not seem right, try your 8 ohm speakers on the 4 ohm taps. You won't hurt anything! What will happen is the impedance dips will me more easily taken care of. You might loose some power, but you will gain sonic performance. Impedance dips can attenuate certain frequencies. Negative feedback tries to compensate for these impedance issues, but sometimes it's better to let the transformer do some of the work.

All I can say is: Give it a try.

[Listens2tubes]

I have used the 4 ohm taps on my MkIV's for many years after reading something similar. When I tried this, on a Marantz 8b amp I switched into my setup, I found it sounded terrible and went back to the 8 ohm taps.

[TRM]

probably the best "tweak" I've ever tried.

[dangerbird]

Same here,,I have a pair of Polk SDA's that are 6 ohm rated,,I tried both the 8 and 4 ohm taps,, the 4 ohm works better for me.