Is a certain amount of hum normal?

I got a Stiletto Ace combo and needed to retrun it because of a very loud hum coming from the speaker even with everything unplugged. My dealer was nice enough to return it and get me a brand new one. This one is not as loud but still does it. I am wondering if they all do this. It is not loud but you can hear it. It does not increase with the amps gain or volume so if you are playing at normal leveals it does not dorwn out the sound by any means. I have just had some bad luck with amps and if there is something wrong I want to get it fixed. I am wondering if it is just the fan being picked up by the speaker or something like that. It did have a bad tube when I picked it up so my dealer replaced all of them for me. Thanks