Mike
I also measured the frequency response of the Toroid before I used it in the Telefunken. It was was pretty good to well over 20kHz. The owner of the Tape recorder said it sounded just like it had always done so.
The difference between a mains transformer and an output transformer is the method of winding to keep the leakage inductance down. Have a look at how the Williamson Output Transformer was wound in the following link and that will give you an idea why a decent output transformer costs a fair bit..
http://www.xs4all.nl/~ideas/amps/chapt7.html
I spoke to the people who took over Avel Lindberg today and they confirmed the primary of their transformers is bifilar wound so when you connect the two primaries in series there is only the enamel insulation between centre tap and what would be the Valve Anodes.
Denis