The laminations are typically very thin, high quality silicon steel strip, the dual windings for the primary and secondaries are precisely controlled, the stepdown ratios, particularily for the lower voltage transformers, are in the same range of those of output transformers at speaker level (8 ohm output impedance). The only drawback is they are intolerant of DC, and therefore you need to make sure you have tight balance between the tubes. There are several companies that make toroidal output transformers (Piltron, Toriody, etc.). But from an experimenter's standpoint, a small toroidal power transformer may do just fine for a few watts output in PP. My recollection is that Richard Sears used a 6AS7 tube for a 4W amplifier.
http://www.triodeguy.com/6as7_pp.htm