Don't really know hot this attenuator works, as is intended to be used trough the effects loop.
I built some time ago an attenuator for my tiny Black Heart. This one is set between the amp's head and the cab.
It just uses a l-pad (15W, 8ohm for the black heart) and a 47uf bipolar cap.
Very simple build:
You can get all the sound of the valves without tearing down the walls of your room :)