My buddy had one. We made lots of mic'd clips comparing both through the same cabs.
They are super close. SLO bit more sweet, for lack of a better term, MZero more mid range grind. A Klone into the SLO crunch channel was near identical to the boost on the Mzero.
Apparently the Steckel has more gain, with a thicker eq. He runs it with the drives, bass and mids maxed, in any pics I can find. I can see that on the drive channel, I do that a lot and boost it when I want a tighter, Marshally sound.
That's A LOT of gain on the overdrive channel, even with lower output pups and volume rolled back. Crazy.