toroidal power transformers?

Elizabeth is correct. And yes, as with all products of the world there are quality differences. Toroidals are not necessarily better or worse than other designs. Some of the best "iron" is used in the old Dynaco 70. My understanding is that David Hafler performed extensive research on eddy currents and overall interaction with AC and transformers.

The best iron isn't iron at all, but metallic glass! It's too pricey for most transformers though...

Although this two-part article is arguably somewhat biased, it does a pretty good job of describing the tradeoffs that are involved between the leading types of power transformers:

http://bryston.com/newsletters/84_files/vol8is4.html

http://bryston.com/newsletters/85_files/vol8is5.html

Regards,
-- Al

Al, that was somewhat biased.

Ayre and Onkyo are exceptions for their own reasons.