toroidal power 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.