Are amp transformers wax potted?

Wax potting (similarly epoxides or polyester resins) do reduce transformer hum, but don't completely stop it.  I wouldn't like to blindly assert that they have no effect on the sound quality of an audio amp, but they have no significant effect on the simple electrical characteristics of the transformer.

I don't know if Naim transformers use any form of potting on the windings, but the torroidal core is almost certainly bonded (typically epoxides or lacquer are used).