Bonebuster, that head is pretty stubby. Some of the torque of the single bevel is created by the length of the blade, the longer the blade the more torque it creates. My guess would be that is the reason you didn't get the "S" cut.
I've applied single bevels to some of the stubbier Magnus heads, and while they rotated I did not get an "S" entry either. I do get the "S" with longer heads like Grizzlies, Tuskers and even the Zwickey No Mercy, although it is not as pronounced with the Zwickey as the other two.
As far as the Abowyer's go, they look like a great head and I've not heard a single negative about them. I'll have to give one a test someday.