From Wikipedia:
Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and greater choice of shapes. The main disadvantages are higher cost and limited rating.
Google Toroidal Transformer, there's several mfgs with good explanations on their websites. They're used in medical equipment, sensing devices, all kinds of high end electronics...not just audio.