??slight hum from the speakers ?

System:
Usher mini-dancer 2 speakers
Emotiva XPA-1 monoblocks, Emotiva ERC-2 CD player
Mystere ca21 pre-amp

Just received the Mystere pre-amp & noticed a slight hum when nothing is playing, compared to dead silence with my other Emotiva pre-amp UPS1

Is it to be expected with a valve pre-amp??