Transformer – Size Matters.
As a rule of thumb, big transformers are an indication of power. I look at the output transformer (always the smaller transformer, sometimes it’s mounted underneath the chassis with the circuitry). A good power transformer doesn’t have to be real big, they are only pushing 40 or so watts, but a real tiny output transformer means a thin sound to me. It is nice if the output transformer is almost as big as the power transformer. It would be nice if the output transformer could be even about half the size of the power transformer.
Some amp techs take output transformers from big amps and put them on small amps. This seems to give more "balls" to amp, without increasing it’s measured power output.
Power transformers are in a "can" which makes them look bigger than they actually are. Output transformers are most often not "canned" so you see the paper and perhaps even the wire windings.