Very high sensitivity speakers (like yours) will easily reproduce ANY noise present in your system. Tube amps and preamps generally have some hiss present and occasionally some hum too. You might try lifting the ground with a cheater plug on the amp just as an experiment but it sounds like the hum is low in level and normal given your mix of equipment. If it's not too intrusive I wouldn't worry about it. The other speakers you were using were not likely as efficient as your current set which is why you didn't hear the hum.
