It's still an issue of power output. Some (mind you, not all) SE amps have their design just right--by choosing the driver tube and circuit properly, it's possible to get some distortion cancellation. Because this is VERY difficult to design for (computer models and plate curve analyses aren't accurate enough), it's more a matter of finding the right combination. Trial and error.

The WE 91A is like that. I suspect that a low-mu triode, combined with a driver tube having very low distortion and a plate characteristic similar to the power tube (310A pentodes of all things), offers the best chance at effective cancellation.

One amp I've seen that does it is the DeHavilland 845 monoblock. Kara did a LOT of trial and error, and came up with something rare. (Please, don't lecture me about whatever the Sound Practices crowd is into this week, or what Bottlehead sells. That stuff is all over the place with regard to cancellation effect.) I feel that SE is popular primarily because it is easy for DIYers to build and mess around with.

The safest way is still push-pull. It guarantees cancellation of even harmonics. If you have a lot of patience and really accurate test equipment, you can try for a cancelling SE circuit. Lots of luck.