Never noticed the hum, although just stuck my ear next to it, there is a slight hum, but this can be solved fairly quickly, I think there are some details on the little dot forum. But I shouldn't worry too much unless you have ultra sensitive hearing. I think this may be reduced in my case with the use of a mains conditioner, unsure though.
You will enjoy it very much, I'm using mine as a pre-amp as well, it really is good value for money, will be doing some tube rolling soon, can't wait to have a play. Give it a good run in, I noticed significant improvements after 40 or so hours, and it just gets sweeter. From what I'm reading the unit its self takes a while to mature due to the large capacitors, but its worth the wait. Post impressions after you have had a listen.