The kind of attenuator I am talking about is inserted between the amplifier's output and the speaker. Turning down at the mike and up at the amp simply will not drive the amp hard. The volume knob on the amp may be at 11, but if you have turned the mic volume down to the point that the amp is only being fed a tiny signal, you will not be driving the amplifier into the zone where the distortion that many of us find pleasing is produced. Attenuators will not result in the speaker being driven hard, an additional component of distortion. I count myself fortunate in that it seems the amps I use crunch at low volume settings.
Attenuators are primarily for larger amps.
I have not used them in conjunction with tube substitutions, but there is no reason not to. Attenuators, I believe, inevitable mess to some degree with tone. The less attenuation you have to use, the better, and the right tube substitutions could reduce the degree of attenuation needed. I have not used an attenuator with a small amp. None of the really small amps I have tried would, in my opinion, benefit from the use of an attenuator except in very quiet practice or acoustic jam settings.