I have a Dr. Z Airbrake that works pretty well with most of my amps. I don't really use it with my Hagen. I find that netween the channel and master volumes, i can tame the beast.
Well, even though I managed to get one of the illusive Phaustine Fantom's from the factory before they went belly up, I dare say I wouldn't use it with my Herbert. As others have said, you have many volume controls available on Herbert and should have little problem getting it to be more quiet. I found great tones from my Herbert at lower volumes. Yet, cranked it does scream
So, should you really feel the need to use an attenuator, I have heard great things about the Rivera Rockcrusher. Line out, adjustable ohm rating, variable attenuating down to "bedroom" levels. Some say they are on par with the Fantom, which is the best I have ever heard. Sold my airbrake, which was also decent.
Anyway, you might give the Rockcrusher a try. I would like to try one, yet I stand with others who will discourage any attenuator use with ANY Diezel amp