attenuation

It may have sonic effects, but those effects may be positive. 

I have a similar issue. The attenuators themselves have a sonic fingerprint. Sit down, relax and try a couple of different settings. You may hear no difference, you may hear some. Only one way to be sure ...

Trust your ears. 

I would be inclined to engage the 20dB attenuation via a fix resistor to reduce the attenuation provided by the variable resistor thus allowing better volume control. My thinking is that this would allow greatest sound quality, wether detectable or not.

I am firmly in the  measure AND listen camp with the understanding that science moves.....including our understanding of the ear brain

have fun, enjoy

Brief correction the cantons are 120/200 watt range in power handeling.

Tomic thanks for this advice you are correct it is all about compromise I'll side for now with the ability to use my attenuator options as I can't hear any distortion.
I believe a big part of our problem is we take ourselves too seriously.
Case in point  I noticed a post the other day that was pulled that got into the science vs what you hear debate folks get too emotional. Both camps have valid points and I appreciate both points of view. 

Careful who you pick as a guru, many manufacturers mechanical step potentiometers, very few including in the high end build a discrete resistor stereo volume control. See Ayre KXR for an excellent example. No device is free of artifacts, which do you prefer ?

Russ  Thanks just needed reaffirmation that it does not add any distortion. 

MC as always you are quick to help rookies with your insightful thoughts.
To answer your question both. The 20 dB lets me make finer adjustments with the other. 
But I'm confused my amp goes to 100 well beyond 11( ventage gear I guess) and when It does my body starts shaking all over and my spine feels tapped out from all that jitter. But not to worry I bought an external Muon stabilizer( only ten grand) direct from the manufacturer and it comes with a 30 day in-home trial however the shipping cost was a killer. I am happy to report It performed as advertised and remove the jitter I was feeling and now all is well in a new and unified cosmos.   

It's fine either way but the system is right in front of you, do you hear any degradation in sound with the attenuator engaged? I would not think you would. Flip it on and enjoy or flip it off, no matter. 

The knob you are calling a volume knob is in reality an attenuator. The mute or 20dB switch is also an attenuator. Both are really just a resistor. The volume knob is probably a trim pot, short for trim potentiometer, a fancy word for variable resistor. If the volume knob clicks then it could be a stepped attenuator, a fancy term for a knob that lets you select one resistor after another until you find the one with the volume level you want. 

That is really all you are doing with all this twiddling around, finding the resistor value that yields the volume level you want. You can do this with the volume knob alone or you can do it with the volume resistance plus the 20dB resistor. 

Which do you think is better? Running the power through one resistor or two? One set of switches, or two? So there's your answer. 

Sorry but it has been scientifically proven no one can fully grasp the cosmic importance of where the volume knob is pointed until they have studied this documentary video produced to get the full impact across.  https://www.youtube.com/watch?v=4xgx4k83zzc