Im not familiar with this particular bit of kit however I can tell you that this has built in preamps controlled by the gain pots.

If this thing is an amp, not having a master volume is very strange.

You have described this thing as a combo so I'm presuming it has the mixer, speakers and amp all in one box, is my presumption true ?

Also Im thinking that if you can hook up both guitars and a vocal line and it sounds OK than it is job done !