I have some concerns about SE. The E-Linear circuit was developed *ABOUT* in parallel( and from the same inspiration ) for SE by a guy named Peter Millet.Plate-to-grid NFB and power supply simplification are some of the benefits. The 813 has a comparable plate rating to the other TX triodes mentioned, and will make more power when run U-L or as a pentode.
It has an anode cap which has historically been a bit intimidating to DIY-ers.
I put a hot link to Pete's site. THere is a PDF download on the E-Linear amp( among others ). Enjoy!
regards,
Douglas