The Freya is known for this problem. Do some research and you will find that you are not alone. You might be able to find a better tube but you also might be chasing your tail. I have had 6SN7s prove noisy in Schiit products that are not noisy in all other applications and for this reason I will not retube any Schiit product. I have even hard clients that had Schiit "selected" tubes go noisy in a very short period of time. I had heard that they improved this component in this regard with later production units.
BTW humming and buzzing are not typical indications of a microphonic tube. Ringing and howling are more typical.