Al,
I think that a good part of what contributes to a blank around 10 feet working for you in single and double hand situations is:
1) The obvious handle configuration - so you can hold it however and there isn't so much extra cork it makes single handing a chore with respect to weight or awkwardness.
2) More importantly, you have a longer bellied weight forward type line or a double taper line. Here is why I believe this.
I have an SA WF (Longer belly WF though - I think 58-62 feet to the back of the rear taper?) Steelhead Taper. This line casts quite well on my 6 wt. 10 foot Sage RPL+ when I am doing single handed spey casts. It particular does the snake roll on small streams well out to about 60 feet or so. I also have a clear intermediate SA, now discontinued but supposedly going to be replaced with something along the lines of a "west coast taper," Striper taper. While the striper taper is a full intermediate line which will influence how it casts somewhat to that of my floating line mentioned above, Still, I have tried casting it's short compact weight forward head with spey casts and it doesn't work well at all. Part of this I feel is becuase of the short compact head, and part of it also has to do with the short rear taper as well.