About a year ago I went looking for a guitar to keep by my chair in the living room, I wanted it to play well, but not cost too much as it will be in the living room where people and kids walk by. I found an Epiphone Les Paul Studio. Having played on it for about a year now, I've grown to like the thing and I'm surprised how nice it plays. I would not go buy another just to get the Gibson name on the head stock. I have only plugged it in an amp 3 or 4 times, but I have always been happy with its sound.
I’m sure Amp’s are the same way.
If the Mustang does not have a line out in the back for speakers it was most likely not built to power a 4 12 cabinet. The head phone jack for sure was not. Well it might for a “short” period of time, but not for a gig. My guess is it would hurt the amp.