I've sharpened the single bevel Abowyer, Grizzly, and Zwickey no mercy with no problem. Here's how I've done it" I first work the edge with a good, new bastard file - working off most of the machining marks until I can feel a good edge. Be very, very careful to maintain the original angle along the entire length of the blade. Then a few strokes on a medium flat diamond hone - mostly to the bevel side with a touch up to remove the burr on the flat side - until it begins to shave, finished on a steel or ceramic stick - again, mostly working the ground side, a few strokes on the "back" side. I can generally get a shaving edge in a few minutes per head. If I do get a head where I run into problems, I start over with the file, carefully working at the original angle.
I've found the narrow edge angle of the single bevels to get a very sharp shaving edge. The problem with many of the double bevel heads, if they are made of sandwiching two pieces of steel together, is that the cutting edge is right where the two pieces of steel are joined - not a problem on single bevel.