Do all toroidal transformers hum?

It doesn't sound normal to me, at least I have never tried it. The bigger question is "do you often listen this way". If not I wouldn't worry about it.

The question is, does the slight hum have any practical effect. While the best transformers may not hum having one that makes the slight noise yours does does not mean that it is a bad transformer or that performance is necessarily bad. There are components where everything has been selected with cost no object and they are very expensive. Good designers select components that will do a good job at a reasonable price, if you are happy with the performance otherwise don't worry.

You may also help by using either coupling or isolating devices under your amp. Although they work on opposing principals I have found both to improve the sound to a surprising degree. I have been using Audio Points from Star Sound but am now using FIM isolation devices [modified] under all components. But there are many others.