I have been making transformers for about 55 years. I dont know all that much about SE traffos as I have NEVER had any real success with them. They are inherently lossy and with the core unbalanced it will NEVER be linear. I suppose one could reduce that somewhat by uses a stupidly large core, so that the DC bias would not be completely overcome the linearity.
SE does work, and it works well for kitchen radios, and portables because its lightweight, and its cheap to make. I would NOT say its Hi-Fi though, and I use Williamsons origional specification ( he DID coin the phrase after all ) 20 Hz to 25 Khz with less than 1 dB variation over the bandwidth at rated full power. Remember too that he used 807's in the origional amplifier, strapped as triodes, and running a big 15 watts to keep everything working at best linearity.
I know I will get arguments/discussion about this but read ANY decent electronics theory on transformers and trying to make them work with DC bias on them. I have already said that toroids dont do well ( MY experience only ) as they saturate very easily if the BH curve is upset.
Apparently they do OK in push/pull operation, but I have never tried it myself. Coupling would be excellent and capacitance pretty large due to the excellent coupling.
As far as DC bias goes, microwave ovens are half wave rectified and the power transformer has all sorts of "gimmicks " to prevent it saturating completely. Namely a big wedge of leakage designed into the traffo itself to prevent it saturating. PLUS, so far I have never seen a toroid in a microwave oven, always EI and always quite poor by Hi-Fi standards, the iron used. Its mostly 3/4's of a MM thick and even 1 mm thick in large commercial types. I use .35mm or 14 thou in my transformers, and if you want to pay extra, and put up with the wait time I will wind with 7 thou or .178 mm for really low leakage and wide response.
Even chokes designed to smooth HT and reduce ripple have the same specs. The BEST chokes are made with 7 thousands of an inch thick laminations.
All the above is just my experience.
Joe