with the swicthing jack i show on the schem/layout, you can use the amp as a combo by removing the plug and reconnecting the speakers. no dummy plug needed.
as for the mid control, what i did was remove input 2 and moved the volume to to that hole, and rewired a mid control in the tone controls by sliding the bass knob where the volume was and placing the mid control where the bass was. this way when used as a combo the only function i lost was the use of input 2, which i never use.
" The more elements you have stable, the more you can concentrate on your playing and not be continually adapting your technique to your equipment." ~ Jerry Garcia