Como nadie me contestaba, hice la consulta a Mesa Boogie, y me acaban de contestar esto:
Thanks for your email. If you are using just one side of the 20/20 you will want to run from the 8ohm output of the side you are using and for the side that is not being used you just want to make sure you turn the master all the way down and the presence all the way up. This will help prevent issues to not having a speaker load connected to one of the sides.
Es curioso porque me dicen, o eso entiendo yo, que para el canal que no tiene carga, el master (supongo que se referira al volumen) abajo y la presencia arriba.
No veo la relacion, pero si ellos lo dicen así será.