There's also the losses in the transformer laminations. Better quality steel would have lower losses, but would be more expensive. The transformer would not get as hot while it is powered but unloaded. Probably less hum.
If Scott stops by I am sure he can tell us about a whole lot of things that affect "loss" in a transformer. I have bumped around the edges of this stuff but just enough to understand this is more than 2 coils of wire and a core when you get down to the nitty gritty of the difference between the "goesinna" and the "comesoutta". The only thing I know is the loss ends up being mostly heat so the temp this thing runs at is a pretty good clue about the efficiency.