I’m resurrecting my M13 for an acoustic project, Whilst I could use the GT1, I don’t really need amp modelling and the reverbs/delays of the GT1 don’t hold a candle to those of the M13.
The M13 control can be considerably (and easily) expanded by adding a pair of expression pedals, such as the remarkably cheap (£14 new) EX-P.
Trouble is, these two devices don’t talk to each other out of the box. But, there’s a simple mod to make them talk. Cut the stereo cable by the plug and connect the two smaller wires (not the earth) to the cables of a standard mono jack plug. Hey presto! The warranty will then be void, but at that price, who cares?