The Hughes & Kettner Red Box is a direct box with filter circuitry designed to emulate the sound of a guitar speaker cabinet, hence its "Cabinetulator" moniker.  It includes a switch to select between the emulation of a 4x12 cabinet or a combo cabinet. 
The Red Box can accept line outputs or speaker outputs, but if you opt for the latter:
If your guitar amp has a tube power stage, you MUST use a speaker or passive load in the THRU jack. 
The Red Box does a fine job of emulating the sound of a guitar speaker cabinet. The 4x12 setting sure sounds like a Marshall cabinet, and the Combo setting does sound like an open back cabinet associated with combo amps. The box can be powered from battery, from AC adapter, or phantom power if your console is equipped. Going direct into a recording system offers the maximum flexibility for processing that isn't possible with a guitar rig.