Impendance taps setting on a tube amp

From Roger M:

"The RM-10 is very tolerant of short circuits and will not be damaged by them." "The amplifier is flat within 0.1dB and has low distortion of 0.3% when played below clipping on average level material." "At the recommended bias current of 30mA/pair, the idling dissipation is nine watts or 75% of the tubes' rating. I estimate tube life to be 5,000 to 10,000 hours. Although higher idling currents will reduce distortion, it can also be reduced by light loading. Basically, light loading reduces the output current demand on the output tubes, allowing them to be more linear. It also reduces noise, raises damping factor, reduces distortion by 78% and allows for 80% more peak current when needed. The only loss is about 20% of the power rating or 1dB."

Thank you all for interesting responses. I'm not sure why RA decided to keep the taps inside, but I assume there must have been a good reason for it. Here's an excerpt from the Cronus manual regarding the impedance setting:

The Cronus also provides options for either 4 or 8 ohm speakers. The impedance selected will depend on the speakers used. Consult either the owner's manual for your
speaker or your local dealer to determine the correct choice. The amplifier is factory set to 8 ohm.
To switch impedance setting:
1) Using a #2 phillips screwdriver, remove the top cover or deck.
2) Using a 5/16” wrench, remove the outermost nut from the positive (red) binding post.
3) Remove the 8 ohm tap (green wire marked 8) from the binding post. Be careful to leave the black wire with heat shrink on the post.
4) Remove the polyurethane tubing from the unused 4 ohm wire (yellow) and lug.
5) Place the 4 ohm wire onto the binding post and replace the outer nut.
6) Place the polyurethane tubing on the 8 ohm wire and lug.

Not looking forward to fiddling with this, I must say. I just hope it sounds more complicated than it actually is...

Another point I wanted to make and see what you guys thought about it is the fact that most speakers' impedance is not linear but varies across the frequency spectrum. Out of curiosity I went through a few copies of Stereophile yesterday and checked the graphs in the tech specs section. All of the speakers reviewed including ones in the $30-40K price range exhibited impedance that was both above and below the specified nominal impedance. If that's the case, wouldn't the arguments about "light loading," considerations about losing power or halving power be rather moot in practice, given the fact that the amp is presented with different loads depending on the frequency?

Wouldn't the arguments about "light loading," considerations about losing power or halving power be rather moot in practice, given the fact that the amp is presented with different loads depending on the frequency?

My feeling is that those considerations are not made moot by variations in impedance vs. frequency, they are just made less predictable, and the need to judge by listening is given greater impetus.

Since maximum power levels are typically required in the bass region, though, I suppose that some sense of the degree of power sacrifice that would result from "light loading" could be had by assessing the degree to which the speaker impedance in the lower part of the spectrum is greater than the impedance that the tap the speaker is connected to is nominally intended for (if in fact it is greater!).

Another point to keep in mind, though, related to that part of the spectrum, is that for a given amp design output impedance is lower at the 4 ohm tap than at the 8 ohm tap. Everything else being equal that will result in better damping of back emf from the woofer, and tighter control of the woofer, as Phil indicated in the first response above. So that is one more tradeoff that is involved, along with the others that have been mentioned -- reduced maximum power vs. improved woofer control.

Regards,
-- Al

So what was the verdict? How did each tap sound and which was better?

I settled for the 4 Ohm setting, but didn't listen long enough with the 8 Ohm setting to provide meaningful feedback. I started with the factory wired 8 Ohm setting and thought the volume wasn't loud enough so I assumed the 4 Ohm setting was going to be better. That was after about an hour of listening. The 4 Ohm tap sounds great so I didn't go back to comparing. The difference between volume levels is almost negligible but I think the 4 Ohms setting provides an overall optimal performance.