[GrimJosef]

What happens if you disconnect the feed back resistor and capacitor, R11 and C6, at one end ? In voltage terms that should make the amp's sensitivity 20 times higher and any hum-and-noise 20 times worse. If it doesn't then either the feedback components (the above two plus R3) aren't working properly or the noise is arising outside the fedback loop or, as Dave says, something might be oscillating.

EDIT: You might think that because you can't see any oscillation at the speaker, there isn't any. But it could be that it simply isn't getting through the output transformer, which isn't designed to pass the sorts of extremely high frequencies characteristic of amp instability.

Cheers,

GJ