I got this amp from a friend of mine He just asked me one day when I was doing some work at his house if I wanted it and he gave me it and the Big M cabinet that he bought with it back in 1973 or 74 The cabinet has AlNico magnet rolla speakers it was not functional, he said the last time it was plugged in smoke came out well I replaced all the caps inside of the amp (all leaking) and the dry rotted power cord and when I fired it up it sounds pretty dang good and when you add a little EQ pedal to it it's even better has a lot of good tones to it. I wasn't sure about the slave output but I saw on the board where it says mixer out.

I wasn't sure about turning the cab ohm selector switch to off (but why would they have it if you couldn't use it right)

I wasn't sure about this I didn't know you could run solid state amps without a speaker so that's good to know thank you so much for the info.

I've had no luck trying to find a owner's manual... My question is on the back of the amp there is this slave \ direct INJN and the output ohm selector switch has an off position, also on the main board where the wire comes from the slave connection to the board the board says "mixer out"...
So the question is can I turn the output selection to off and use the slave connection to go into another amp or mixer without having to connect a cabinet?

I was even thinking possibly using it as a preamp before another tube amp just to see what it would do.

I added the send and return jack picture just show that the other is not an input or part of the sending return.

Couldn't figure out how to add pictures to my post so I gave the link to where I asked the same question in Marshall forum so if you wanted to see the pictures you can look there thanks in advance.

I posted this same thing in Marshall forum but I get no answers I couldn't figure out how to post pictures here