Nope, that is not an attenuator, even a simple one, it's just a good quality volume potentiometer with a long adjust and . Attenuators come after the power amp and dissipate their energy somewhere thus reducing the output, but allowing / forcing the power tubes to work hard. You can do exactly what that box does by putting any volume control box in the loop, like a multifx unit, a pedal with volume control or EQ pedal or whatever and it just allows for precise changes in sounds, which can of course be good in that many tube amps have precious little room for any volume adjust, they go from zero to 100db often in the first tiny nudges of the knob. (that sounds kinda dirty to me btw
However, in terms of why attenuators are used, that volume control device does pretty much exactly the opposite of what they try to achieve, in that it will reduce the power tube gain thus producing a cleaner and colder sound and attenuators try to get more gain and compression and warmer sound without the resulting db increase.
FWIW, I don't recommend any L-pads or brake style attenuators with any powerful tube amps; on 5 to maybe 15W amps they might be sufficient but are all ultimately bad for the amp, the worse the more attenuation is used. A simple L-pad circuit works fine for like a 5W tube practice amp and a better designed like Weber mass-power brake likely works fine for little more power, but if you attenuate, say a 50W amp regularly and heavily, it _will_ blow up at some point quite catastrophically due to no variations in resistance etc. don't remember exactly what gives but google it, it's not a good idea for longer/heavy use.
There are a few makers of true power soakers with variable resistance, i.e. effectively compressed speaker coils with mass but no cone, and those are AFAIK the only reliable and safe way to attenuate a full-blown tube amp...they're just insanely expensive.
